docs: document 'list' behavior when 'listchars' excludes "tab" (#29360)

src/nvim/options.lua

@@ -4876,6 +4876,9 @@ return {
         between tabs and spaces and for trailing blanks. Further changed by
         the 'listchars' option.
 
+        When 'listchars' does not contain "tab" field, tabs are shown as "^I"
+        or "<09>", like how unprintable characters are displayed.
+
         The cursor is displayed at the start of the space a Tab character
         occupies, not at the end as usual in Normal mode.  To get this cursor
         position while displaying Tabs with spaces, use: >vim