precision on email_filter

The filtering regexp should not allow greedy matchers (such as .*) but
use non greedy ones (such as .*?).  One want to escape dots as well.

Additionally, whenever one want to add several emails, he should use a
YAML list.

Change-Id: I9deea0893151358ddde9b1ec5a40c322c45d7ff2
Reviewed-on: https://review.openstack.org/18350
Reviewed-by: James E. Blair <corvus@inaugust.com>
Approved: James E. Blair <corvus@inaugust.com>
Tested-by: Jenkins
diff --git a/doc/source/zuul.rst b/doc/source/zuul.rst
index 25c2087..55e0201 100644
--- a/doc/source/zuul.rst
+++ b/doc/source/zuul.rst
@@ -254,7 +254,10 @@
 
   *email_filter*
   This is used for any event.  It takes a regex applied on the performer
-  email. Example: ``email_filter: .*@example.org$``.
+  email, i.e Gerrit account email address.  If you want to specify
+  several email filters, you must use a YAML list.  Make sure to use non
+  greedy matchers and to escapes dots!
+  Example: ``email_filter: ^.*?@example\.org$``.
 
   *comment_filter*
   This is only used for ``comment-added`` events.  It accepts a list of