PushToGerrit: Improve tooltip

Task-number: QTCREATORBUG-11930
Change-Id: Id09b71759a1fe67c558500a2a460669f708b1dee
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@digia.com>

src/plugins/git/gerrit/gerritpushdialog.ui

@@ -154,6 +154,8 @@
        <property name="toolTip">
         <string>Comma-separated list of reviewers.
 
+Reviewers can be specified by nickname or email address. Spaces not allowed.
+
 Partial names can be used if they are unambiguous.</string>
        </property>
       </widget>