doc: tag: document `TAG_EDITMSG`

doc: tag: document `TAG_EDITMSG`

Document `TAG_EDITMSG` which we have told the user about on unsuccessful
command invocations since commit 3927bbe9a4 (tag: delete TAG_EDITMSG
only on successful tag, 2008-12-06).

Introduce this documentation since we are going to add tests for the
lifetime of this file in the case of command failure and success.

Use the documentation for `COMMIT_EDITMSG` from `git-commit.txt` as a
template since these two files share the same purpose.[1]

† 1: from commit 3927bbe9a4:

     “ This matches the behavior of COMMIT_EDITMSG, which stays around
       in case of error.

Suggested-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Kristoffer Haugsbakk <code@khaugsbakk.name>
Signed-off-by: Junio C Hamano <gitster@pobox.com>