git blame: document that it always follows origin across whole-file renames

git blame: document that it always follows origin across whole-file renames

Make it clear to people who (rightly or wrongly) think that the
"--follow" option should follow origin across while-file renames
that we already do so.  That would explain the output that they see
when they do give the "--follow" option to the command.

We may or may not want to do a "--no-follow" patch as a follow-up,
but that is a separate topic.

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