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

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>