intro-user.txt: Mention that topics may affect submission

Change-Id: I25b056bce62e11b5168c8a2ad2dda5a8e554a639

Documentation/intro-user.txt

diff --git a/Documentation/intro-user.txt b/Documentation/intro-user.txt
index 6242fcf..c2d53e0 100644
--- a/Documentation/intro-user.txt
+++ b/Documentation/intro-user.txt
@@ -474,6 +474,10 @@
 the use of the command line flag `--push-option`, aliased to `-o`,
 followed by `topic=...`.
 
+Gerrit may be link:config-gerrit.html#change.submitWholeTopic[configured] to
+submit all changes in a topic together with a single click, even when topics
+span multiple projects.
+
 .Set Topic on Push
 ----
   $ git push origin HEAD:refs/for/master%topic=multi-master