Documentation: do not mention .git/refs/* directories

Documentation: do not mention .git/refs/* directories

It is an implementation detail that a new tag is created by adding a
file in the .git/refs/tags directory.  The only thing the user needs
to know is that a "git tag" creates a ref in the refs/tags namespace,
and without "-f", it does not overwrite an existing tag.

Inspired by a report from 乙酸鋰 <ch3cooli@gmail.com>; I think I
caught all the existing mention in Documentation/ directory in the
tip of 1.7.9.X maintenance track, but we may have added new ones
since then.

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