git-codereview: update help text to add -trybot

It was added to the godoc in https://golang.org/cl/24960, but that CL
missed the help text given in "git-codereview help". Briefly mention
it there too.

Change-Id: I6905b3f98f4092ed34609596bb875e88cb00d480
Reviewed-on: https://go-review.googlesource.com/41693
Reviewed-by: Ian Lance Taylor <iant@golang.org>
diff --git a/git-codereview/review.go b/git-codereview/review.go
index f4d0096..ab5a795 100644
--- a/git-codereview/review.go
+++ b/git-codereview/review.go
@@ -82,7 +82,7 @@
 		Every other operation except help also does this,
 		if they are not already installed.
 
-	mail [-f] [-r reviewer,...] [-cc mail,...] [commit]
+	mail [-f] [-r reviewer,...] [-cc mail,...] [-trybot] [commit]
 		Upload change commit to the code review server and send mail
 		requesting a code review.
 		If there are multiple commits on this branch, upload commits
@@ -91,6 +91,8 @@
 		The -r and -cc flags identify the email addresses of people to
 		do the code review and to be CC'ed about the code review.
 		Multiple addresses are given as a comma-separated list.
+		If -trybot is specified, the trybots are run on the changes,
+		if permitted.
 
 	mail -diff
 		Show the changes but do not send mail or upload.