Update and clarify StackForge documentation.

* doc/source/stackforge.rst: Release tagging seems to be a FAQ
amongst StackForge projects, so I've added core tag permissions to
the example. Setting the project active is unnecessary since that's
the default anyway. The acl-config parameter in project
configuration is no longer needed unless it differs from the normal
file location/name. Also added some further clarification and
explanations for the ACL example.

Change-Id: Id97d9fdf9e99e2948a83aea90c292d63b4c1e4d1
Reviewed-on: https://review.openstack.org/25005
Reviewed-by: Khai Do <zaro0508@gmail.com>
Reviewed-by: James E. Blair <corvus@inaugust.com>
Approved: James E. Blair <corvus@inaugust.com>
Tested-by: Jenkins