git-codereview: document <branch>.mailed tag

In earlier commits I have said it was possible to use <branch>.mailed@{1}
and so on to get at the history of all versions that were mailed.
This is NOT true: the Git reflog does not record history for tags.
Only the most recently mailed commit is available.

Change-Id: I1e4a6ac510a53bfa3f69a64b6ea5446c914882fe
Reviewed-on: https://go-review.googlesource.com/2068
Reviewed-by: Rob Pike <r@golang.org>