Fix in-app links which point users to the User Guide

In 4.6, as far as I can tell, none of the outbound links within the CiviCRM UI which point users to pages within the User Guide actually work. In 4.7 this problem is fixed. But I'd like to fix it for 4.6.

Steps to reproduce:

1. Begin with latest version of CiviCRM 4.6
2. Go to Manage Contribution Pages at /civicrm/admin/contribute?reset=1
3. Find the green help box at the top of the page
4. Click the blue question mark to activate the help pop-up
5. Look at the (learn lore...) link at the bottom of the page
6. Expect the URL for this link to begin with docs.civicrm.org and point to the 4.6 branch
7. Observe that the link URL points to book.civicrm.org instead
8. Expect that clicking the link takes you to a page about contribution pages
9. Observe that clicking the link (after redirection) takes you to civicrm.org/documentation which offers no content specific to this topic.