Does anything document --date=tea/yesterday/noon/midnight/never/now etc.?

These were originally added by Linus back in v0.99.9-230-ga8aca41, a
bit was added to them since then. See
https://github.com/git/git/blob/v2.9.2/date.c#L927

The v0.99.9-214-g3c07b1d commit also has some info on the warty edge
cases these have to deal with.

I was checking what documented "git commit --amend --date=now", but
Documentation/date-formats.txt only mentions ISO/RFC formats.

I thought I'd just submit a trivial patch to document this but a)
maybe it's in some mysterious part of the docs I've missed b) after
reading v0.99.9-214-g3c07b1d and some of the date.c code it looks like
it won't be that trivial to describe this.