doc: am: say that --message-id adds a trailer

The option `--message-id` was added in a078f732 (git-am: add
--message-id/--no-message-id, 2014-11-25) back when git-interpret-
trailers(1) was relatively new. Let’s spell out that it is a trailer
and link to the dedicated trailer command.

Also use inline-verbatim for `Message-ID`.

Also link to git-interpret-trailers(1) on `--signoff`.

Signed-off-by: Kristoffer Haugsbakk <code@khaugsbakk.name>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-am.adoc

@@ -37,9 +37,10 @@ OPTIONS
 
 -s::
 --signoff::
-	Add a `Signed-off-by` trailer to the commit message, using
+	Add a `Signed-off-by` trailer to the commit message (see
-	the committer identity of yourself.
+	linkgit:git-interpret-trailers[1]), using the committer identity
-	See the signoff option in linkgit:git-commit[1] for more information.
+	of yourself.  See the signoff option in linkgit:git-commit[1]
+	for more information.
 
 -k::
 --keep::
@@ -83,10 +84,11 @@ OPTIONS
 
 -m::
 --message-id::
-	Pass the `-m` flag to linkgit:git-mailinfo[1],
+	Pass the `-m` flag to linkgit:git-mailinfo[1], so that the
-	so that the Message-ID header is added to the commit message.
+	`Message-ID` header is added as a trailer (see
-	The `am.messageid` configuration variable can be used to specify
+	linkgit:git-interpret-trailers[1]).  The `am.messageid`
-	the default behaviour.
+	configuration variable can be used to specify the default
+	behaviour.
 
 --no-message-id::
 	Do not add the Message-ID header to the commit message.