git-status documentation: mention subdirectory behaviour

git-status documentation: mention subdirectory behaviour

Consistently with all other diff oriented commands, we have given paths
relative to the work tree root in git-status output for a long time.
This documents the recent behaviour change, as people's eyes (and worse
yet, scripts, although scripts should not parse "git status" output) may
depend on the old behaviour.

In the longer run, giving a --full-name option to git-diff Porcelain
similar to what ls-files has, and change the default for git-diff
Porcelain to show relative paths may be a good thing to do, in order to
hide the oddballness of this git-status behaviour, but that would have a
rather large impact to established expectation by existing users.

Signed-off-by: Junio C Hamano <gitster@pobox.com>