docs: clarify get_current_dir nil return and add telescope recipe

Problem: get_current_dir returns nil in several cases that were not
documented, causing confusion when used in keymaps that open pickers
like Telescope (#682). Also, posix_to_os_path could crash on Windows
when no drive letter is found.

Solution: expand get_current_dir docs to explain nil return cases, add
a Telescope recipe with nil guards, and add a defensive nil check in
posix_to_os_path.

Cherry-picked from: stevearc/oil.nvim#727

lua/oil/fs.lua

@@ -87,6 +87,9 @@ M.posix_to_os_path = function(path)
   if M.is_windows then
     if vim.startswith(path, "/") then
       local drive = path:match("^/(%a+)")
+      if not drive then
+        return path
+      end
       local rem = path:sub(drive:len() + 2)
       return string.format("%s:%s", drive, rem:gsub("/", "\\"))
     else