doc/send-email: mention handling of "reply-to" with --compose

The documentation for git-send-email lists the headers handled specially
by --compose in a way that implies that this is the complete set of
headers that are special. But one more was added by d11c943
(send-email: support separate Reply-To address, 2018-03-04) and never
documented.

Let's add it, and reword the documentation slightly to avoid having to
specify the list of headers twice (as it is growing and will continue to
do so as we add new features).

If you read the code, you may notice that we also handle MIME-Version
specially, in that we'll avoid over-writing user-provided MIME headers.
I don't think this is worth mentioning, as it's what you'd expect to
happen (as opposed to the other headers, which are picked up to be used
in later emails). And certainly this feature existed when the
documentation was expanded in 01d3861 (git-send-email.txt: describe
--compose better, 2009-03-16), and we chose not to mention it then.

Signed-off-by: Jeff King <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: peff

Documentation/git-send-email.txt

@@ -68,11 +68,11 @@ This option may be specified multiple times.
 	Invoke a text editor (see GIT_EDITOR in linkgit:git-var[1])
 	to edit an introductory message for the patch series.
 +
-When `--compose` is used, git send-email will use the From, Subject, and
-In-Reply-To headers specified in the message. If the body of the message
-(what you type after the headers and a blank line) only contains blank
-(or Git: prefixed) lines, the summary won't be sent, but From, Subject,
-and In-Reply-To headers will be used unless they are removed.
+When `--compose` is used, git send-email will use the From, Subject,
+Reply-To, and In-Reply-To headers specified in the message. If the body
+of the message (what you type after the headers and a blank line) only
+contains blank (or Git: prefixed) lines, the summary won't be sent, but
+the headers mentioned above will be used unless they are removed.
 +
 Missing From or In-Reply-To headers will be prompted for.
 +