doc: tag: document TAG_EDITMSG

Document `TAG_EDITMSG` which we have told the user about on unsuccessful
command invocations since commit 3927bbe (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 3927bbe:

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

Suggested-by: Junio C Hamano <[email protected]>
Signed-off-by: Kristoffer Haugsbakk <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: LemmingAvalanche

Documentation/git-tag.txt

@@ -381,6 +381,16 @@ $ GIT_COMMITTER_DATE="2006-10-02 10:31" git tag -s v1.0.1
 
 include::date-formats.txt[]
 
+FILES
+-----
+
+`$GIT_DIR/TAG_EDITMSG`::
+	This file contains the message of an in-progress annotated
+	tag. If `git tag` exits due to an error before creating an
+	annotated tag then the tag message that has been provided by the
+	user in an editor session will be available in this file, but
+	may be overwritten by the next invocation of `git tag`.
+
 NOTES
 -----