docs(pending): document :Pending init and store resolution

Add *pending-store-resolution* section explaining upward .pending.json
discovery and fallback to the global data_path. Document :Pending init
under COMMANDS. Add a cross-reference from the data_path config field.

doc/pending.txt

@@ -1015,6 +1015,26 @@ To create a project-local store in the current directory: >vim
 
 The `:checkhealth pending` report shows which store file is currently active.
 
+==============================================================================
+STORE RESOLUTION                                    *pending-store-resolution*
+
+When pending.nvim opens the task buffer it resolves which store file to use:
+
+1. Search upward from `vim.fn.getcwd()` for a file named `.pending.json`.
+2. If found, use that file as the active store (project-local store).
+3. If not found, fall back to `data_path` from |pending-config| (global
+   store).
+
+This means placing a `.pending.json` file in a project root makes that
+project use an isolated task list. Tasks in the project store are completely
+separate from tasks in the global store; there is no aggregation.
+
+To create a project-local store in the current directory: >vim
+    :Pending init
+<
+
+The `:checkhealth pending` report shows which store file is currently active.
+
 ==============================================================================
 DATA FORMAT                                                    *pending-data*