Careful with Texinfo markup on GitLab

[Jean Abou Samra]

Prompted by https://gitlab.com/lilypond/lilypond/-/merge_requests/1102,
a friendly reminder to everyone because it's an easy mistake
to do: please be cautious when writing about Texinfo commands
on GitLab. If you leave @something without special markup,
it is taken as a mention and random people having accounts
on GitLab with names @qq, etc. get mentioned and start
receiving unsolicited notifications for comments on the merge
request. The typical case is @lilypond, which mentions all 25+
members of the LilyPond group ... This collision is a bit
unfortunate, but the fix is simple: always enclose Texinfo
commands in backticks. For code suggestions, you can also
use the dedicated feature of GitLab, which also makes it
easier for the patch author to apply s/ /  /-style changes. Read
https://docs.gitlab.com/ee/user/project/merge_requests/reviews/suggestions.html
about those. I've also opened
https://gitlab.com/lilypond/lilypond/-/merge_requests/1111
to make new contributors aware of this.

Thanks,
Jean