git-arc.1: Clarify argument format for group reviewers

[bnovkov]

Summary

Note that '-r' expects a hashtag instead of a group name.

[ziaee]

tools/tools/git/git-arc.1

87

Hashtag is from social media and kind of slang. How do you feel about:
"Group reviewers must be prefixed with a number sign
.Pq Ql # ,
e.g. Jails"

Addl. Context: https://en.m.wikipedia.org/wiki/Number_sign

[ziaee]

Maybe we don't even have to bloat it at all and could make it clear by simply saying:
"Each argument must be an existing Phabricator user or #group."

[bnovkov]

tools/tools/git/git-arc.1

87

I opted for "hashtag" since it was already used in one of the examples below, IMO it makes the expected format very clear.

[ziaee]

tools/tools/git/git-arc.1

85–88

Adding a sentence and a parenthetical does make it clear, but it adds a lot of complexity. Would it be clear if we improved the first sentence instead, without adding complexity?

[ziaee]

tools/tools/git/git-arc.1

85–88

[markj]

I think this is fine. A larger improvement would be to sync the usage message and man page, which give similar but not identical content and examples.