Improve and document the PageTitleMixin

[Sam Betts]

https://review.openstack.org/#/c/142802 adds new functionality to reduce duplication in horizon templates, this bug is to track adding extra documentation and readability to those new features.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/154932

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/154932
Committed: https://git.openstack.org/cgit/openstack/horizon/commit/?id=168aa7e1621a05465146738770a83bf40a7f359c
Submitter: Jenkins
Branch: master

commit 168aa7e1621a05465146738770a83bf40a7f359c
Author: Sam Betts <email address hidden>
Date: Wed Feb 11 12:35:27 2015 +0000

    Document and improve PageTitleMixin readability

    This is a follow up patch to https://review.openstack.org/#/c/142802
    that adds documentation and also improves code readability in the
    PageTitleMixin.

    Change-Id: Iac7666252b5ceb9611e0688ef90173edf90475d9
    Closes-Bug: 1420744