vim-patch:9.2.0277: tests: test_modeline.vim fails (#38672)

Problem:  tests: test_modeline.vim fails (after v9.2.0276)
Solution: Rewrite the tests to use the existing s:modeline_fails()
          function, update documentation (zeertzjq).

8c8772c6b3

src/nvim/options.lua

@@ -1543,6 +1543,8 @@ local options = {
         and from tags to 5.  Other sources remain unlimited.
         Note: The match limit takes effect only during forward completion
         (CTRL-N) and is ignored during backward completion (CTRL-P).
+
+        This option cannot be set in a modeline when 'modelineexpr' is off.
       ]=],
       full_name = 'complete',
       modelineexpr = true,
@@ -4255,6 +4257,7 @@ local options = {
         You can include a line break.  Simplest method is to use |:let|: >vim
         	let &guitabtooltip = "line one\nline two"
         <
+        This option cannot be set in a modeline when 'modelineexpr' is off.
       ]=],
       full_name = 'guitabtooltip',
       modelineexpr = true,