runtime(help): add omni completion and 'iskeyword' to filetype plugin

Problem:

- Help tags provide a good way to navigate the Vim documentation, but
  many help documents don't use them effectively. I think one of the
  reasons is that help writers have to look up help tags manually with
  `:help` command, which is not very convenient.
- 'iskeyword' is only set for help buffers opened by `:help` command.
  That means if I'm editing a help file, I cannot jump to tag in same
  file using `Ctrl-]` unless I manually set it, which is annoying.

Solution:

- Add omni completion for Vim help tags.
- Set 'iskeyword' for `ft-help`

closes: #17073

Co-authored-by: Christian Brabandt <cb@256bit.org>
Signed-off-by: Phạm Bình An <phambinhanctb2004@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

runtime/doc/helphelp.txt

@@ -1,4 +1,4 @@
-*helphelp.txt*	For Vim version 9.1.  Last change: 2025 Jan 11
+*helphelp.txt*	For Vim version 9.1.  Last change: 2025 Apr 08
 
 
 		  VIM REFERENCE MANUAL    by Bram Moolenaar
@@ -471,8 +471,13 @@ highlighting.  So do these:
 You can find the details in $VIMRUNTIME/syntax/help.vim
 
 
-GENDER NEUTRAL LANGUAGE
+FILETYPE COMPLETION					*ft-help-omni*
 
+To get completion for help tags when writing a tag reference, you can use the
+|i_CTRL-X_CTRL-O| command.
+
+
+GENDER NEUTRAL LANGUAGE
 						*gender-neutral* *inclusion*
 Vim is for everybody, no matter race, gender or anything.  For new or updated
 help text, gender neutral language is recommended.  Some of the help text is

runtime/doc/tags

@@ -7394,6 +7394,7 @@ ft-groff-syntax	syntax.txt	/*ft-groff-syntax*
 ft-gsp-syntax	syntax.txt	/*ft-gsp-syntax*
 ft-hare	filetype.txt	/*ft-hare*
 ft-haskell-syntax	syntax.txt	/*ft-haskell-syntax*
+ft-help-omni	helphelp.txt	/*ft-help-omni*
 ft-html-indent	indent.txt	/*ft-html-indent*
 ft-html-omni	insert.txt	/*ft-html-omni*
 ft-html-syntax	syntax.txt	/*ft-html-syntax*