Documentation/git-tag.txt: Document how to backdate tags
Added a new section beneath "On Automatic following" called "On
Backdating Tags". This includes an explanation of when to use this
method, a brief explanation of the kind of date that can be used in
GIT_AUTHOR_DATE, and an example invocation of git-tag using a custom
setting of GIT_AUTHOR_DATE.
[sp: Corrected s/you/your/, noticed by Jeff King]
Signed-off-by: Michael W. Olson <mwolson@gnu.org>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>