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>

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