git-codereview: document mail -trybot flag

Change-Id: I86981168df8571e2408ef8bb5c5c0f21e0f3cca6
Reviewed-on: https://go-review.googlesource.com/24960
Reviewed-by: Russ Cox <rsc@golang.org>
diff --git a/git-codereview/doc.go b/git-codereview/doc.go
index ebb1420..d6a5576 100644
--- a/git-codereview/doc.go
+++ b/git-codereview/doc.go
@@ -182,7 +182,7 @@
 
 The mail command starts the code review process for the pending change.
 
-	git codereview mail [-f] [-r email] [-cc email] [revision]
+	git codereview mail [-f] [-r email] [-cc email] [-trybot] [revision]
 
 It pushes the pending change commit in the current branch to the Gerrit code
 review server and prints the URL for the change on the server.
@@ -198,6 +198,9 @@
 from the git repository log to find email addresses of the form name@somedomain
 and then, in case of ambiguity, using the reviewer who appears most often.
 
+The -trybot flag runs the trybots on all new or updated changes. It is
+equivalent to setting the Run-Trybot+1 label from Gerrit.
+
 The mail command fails if there are staged edits that are not committed.
 The -f flag overrides this behavior.