commit: document the temporary commit message file

We do not document COMMIT_EDITMSG at all, but users may want
to know about it for two reasons:

  1. They may want to tell their editor to configure itself
     for formatting a commit message.

  2. If a commit is aborted by an error, the user may want
     to recover the commit message they typed.

Let's put a note in git-commit(1).

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-commit.txt

@@ -399,6 +399,15 @@ This command can run `commit-msg`, `prepare-commit-msg`, `pre-commit`,
 and `post-commit` hooks.  See linkgit:githooks[5] for more
 information.
 
+FILES
+-----
+
+`$GIT_DIR/COMMIT_EDITMSG`::
+	This file contains the commit message of a commit in progress.
+	If `git commit` exits due to an error before creating a commit,
+	any commit message that has been provided by the user (e.g., in
+	an editor session) will be available in this file, but will be
+	overwritten by the next invocation of `git commit`.
 
 SEE ALSO
 --------