patch 9.1.1186: filetype: help files in git repos are not detected

Problem:  filetype: help files in git repos are not detected
Solution: detect */doc/*.txt files as help if they end with a help
          modeline, even if 'modeline' is off

Here's how I checked that this would still detect vim's own help files
correctly:

$ find . -type f -path '*/doc/*.txt' \
> -exec awk '{ } ENDFILE { print FILENAME ":" $0; }' '{}' + |
> grep -v 'vim:.*\<\(ft\|filetype\)=help\>'
./src/libvterm/doc/seqs.txt: 23    DECSM 42         = DECNRCM, national/multinational character

closes: #16817

Signed-off-by: David Mandelberg <david@mandelberg.org>
Signed-off-by: Christian Brabandt <cb@256bit.org>

runtime/doc/todo.txt

@@ -1,4 +1,4 @@
-*todo.txt*      For Vim version 9.1.  Last change: 2025 Feb 21
+*todo.txt*      For Vim version 9.1.  Last change: 2025 Mar 08
 
 
 		  VIM REFERENCE MANUAL	  by Bram Moolenaar
@@ -6276,5 +6276,4 @@ Far future and "big" extensions:
     are reflected in each Vim immediately.  Could work with local files but
     also over the internet.  See http://www.codingmonkeys.de/subethaedit/.
 
-vim:tw=78:sw=4:sts=4:ts=8:noet:ft=help:norl:
-vim: set fo+=n :
+vim:tw=78:sw=4:sts=4:ts=8:noet:ft=help:norl:fo+=n:

runtime/filetype.vim

@@ -1,7 +1,7 @@
 " Vim support file to detect file types
 "
 " Maintainer:	The Vim Project <https://github.com/vim/vim>
-" Last Change:	2025 Mar 06
+" Last Change:	2025 Mar 08
 " Former Maintainer:	Bram Moolenaar <Bram@vim.org>
 
 " Listen very carefully, I will say this only once
@@ -51,8 +51,11 @@ func s:StarSetf(ft)
   endif
 endfunc
 
-" Vim help file
-au BufNewFile,BufRead $VIMRUNTIME/doc/*.txt	setf help
+" Vim help file, set ft explicitly, because 'modeline' might be off
+au BufNewFile,BufRead */doc/*.txt
+	\  if getline('$') =~ 'vim:.*\<\(ft\|filetype\)=help\>'
+	\|   setf help
+	\| endif
 
 " Abaqus or Trasys
 au BufNewFile,BufRead *.inp			call dist#ft#Check_inp()