Add 'doc' tag for bug link on doc theme

This commit adds 'doc' tag for a bug link on the
openstackdocstheme config. This configuration is used for
documentation bug basically. So, this should be good in most cases.

Change-Id: I11b7a9caef7ef896b39c39ddc2d958b6f0891991
diff --git a/doc/source/conf.py b/doc/source/conf.py
index 0a061b8..c2ea628 100644
--- a/doc/source/conf.py
+++ b/doc/source/conf.py
@@ -64,7 +64,7 @@
 # openstackdocstheme options
 repository_name = 'openstack/tempest'
 bug_project = 'tempest'
-bug_tag = ''
+bug_tag = 'doc'
 
 # Must set this variable to include year, month, day, hours, and minutes.
 html_last_updated_fmt = '%Y-%m-%d %H:%M'