docs: update vimdoc to reflect current feature set

Problem: doc/pending.txt was written before the undo stack, folds,
:Pending due, D mapping, and BufEnter reload were added. Several
entries were factually wrong (single-level undo, d vs D key,
:Pending undo listed as non-existent) and highlight group defaults
referenced stale hex colours.

Solution: correct all factual errors and add missing entries —
:Pending due command, :Pending undo command, zc/zo fold mappings,
PendingOverdue highlight group, semantic link defaults for all groups,
category fold docs, BufEnter auto-reload note, and multi-level undo
description.

doc/pending.txt

@@ -33,8 +33,10 @@ Features: ~
 - Inline metadata syntax: `due:` and `cat:` tokens parsed on `:w`
 - Relative date input: `today`, `tomorrow`, `+Nd`, weekday names
 - Two views: category (default) and priority flat list
-- Single-level undo for the last `:w` save
+- Multi-level undo (up to 20 `:w` saves, session-only)
 - Quick-add from the command line with `:Pending add`
+- Quickfix list of overdue/due-today tasks via `:Pending due`
+- Foldable category sections (`zc`/`zo`) in category view
 - Google Calendar one-way push via OAuth PKCE
 
 ==============================================================================
@@ -81,7 +83,8 @@ Buffer-local keys are set automatically when the buffer opens. See
 The buffer uses `buftype=acwrite` so `:w` always routes through pending.nvim's
 write handler rather than writing to disk directly. The `pending://` buffer
 persists across window switches; reopening with `:Pending` focuses the
-existing window if one is open.
+existing window if one is open. The buffer is automatically reloaded from
+disk when entered unmodified.
 
 ==============================================================================
 INLINE METADATA                                             *pending-metadata*
@@ -143,6 +146,11 @@ COMMANDS                                                    *pending-commands*
         :Pending archive 7      " remove tasks completed more than 7 days ago
 <
 
+                                                                *:Pending-due*
+:Pending due
+    Populate the quickfix list with all tasks that are overdue or due today.
+    Open the list with |:copen| to navigate to each task's category.
+
                                                                *:Pending-sync*
 :Pending sync
     Push pending tasks that have a due date to Google Calendar as all-day
@@ -151,9 +159,9 @@ COMMANDS                                                    *pending-commands*
 
                                                                *:Pending-undo*
 :Pending undo
-    There is no `:Pending undo` subcommand. Use the `U` buffer-local key
+    Undo the last `:w` save, restoring the task store to its previous state.
-    (see |pending-mappings|) to undo the last `:w` save while the task buffer
+    Equivalent to the `U` buffer-local key (see |pending-mappings|). Up to 20
-    is open.
+    levels of undo are retained per session.
 
 ==============================================================================
 MAPPINGS                                                    *pending-mappings*
@@ -167,12 +175,16 @@ Buffer-local keys: ~
     -------     ------------------------------------------------
     `<CR>`        Toggle complete / uncomplete the task at cursor
     `!`           Toggle the priority flag on the task at cursor
-    `d`           Prompt for a due date on the task at cursor
+    `D`           Prompt for a due date on the task at cursor
     `<Tab>`       Switch between category view and priority view
     `U`           Undo the last `:w` save
     `g?`          Show a help popup with available keys
+    `zc`          Fold the current category section (category view only)
+    `zo`          Unfold the current category section (category view only)
 
-Standard Vim keys `o`, `O`, `dd`, `p`, `P`, and `:w` work as expected.
+`o` and `O` are overridden to insert a correctly-formatted blank task line
+at the position below or above the cursor rather than using standard Vim
+indentation. `dd`, `p`, `P`, and `:w` work as expected.
 
                                                         *<Plug>(pending-open)*
 <Plug>(pending-open)
@@ -209,7 +221,8 @@ Category view (default): ~                             *pending-view-category*
     order tasks were added unless `category_order` is set (see
     |pending-config|). Blank lines separate categories. Within each category,
     pending tasks appear before done tasks. Priority tasks (`!`) are sorted
-    first within each group.
+    first within each group. Category sections are foldable with `zc` and
+    `zo`.
 
 Priority view: ~                                       *pending-view-priority*
     A flat list of all tasks sorted by priority, then by due date (tasks
@@ -339,20 +352,24 @@ pending.nvim defines the following highlight groups. All groups are set with
 
                                                               *PendingHeader*
 PendingHeader       Applied to category header lines (text at column 0).
-                    Default: bold.
+                    Default: links to `Title`.
 
                                                                  *PendingDue*
 PendingDue          Applied to the due date virtual text shown at the right
                     margin of each task line.
-                    Default: fg=#888888, italic.
+                    Default: links to `DiagnosticHint`.
+
+                                                             *PendingOverdue*
+PendingOverdue      Applied to the due date virtual text of overdue tasks.
+                    Default: links to `DiagnosticError`.
 
                                                                 *PendingDone*
 PendingDone         Applied to the text of completed tasks.
-                    Default: strikethrough, fg=#666666.
+                    Default: links to `Comment`.
 
                                                             *PendingPriority*
 PendingPriority     Applied to the `! ` priority marker on priority tasks.
-                    Default: fg=#e06c75, bold.
+                    Default: links to `DiagnosticWarn`.
 
 To override a group in your colorscheme or config: >lua
     vim.api.nvim_set_hl(0, 'PendingDue', { fg = '#aaaaaa', italic = true })