WL#4183: Cluster: Add Functional Markup to API Docs

Support would like to see names of NDB and MGM API classes, methods, functions,
and so forth linked to their definitions in the Cluster API documentation.

Implementation of this task involves:
(1) Markup: We'll overload 
(2) Modify our XSLT (mysqldoc-toolset/xsl.d/auto-link.xsl)
(3) Add the new markup to the Cluster API Guide, Cluster chapter of the 5.1
RefMan, and changelogs
(4) Document the new markup in mysqldoc-guide

(1) and (2) have largely been accomplished already as a result of WL #3803, no
need to reinvent the wheel.

See http://dev.mysql.com/doc/mysqldoc-guide/en/auto-link-markup.html for some
ideas as to how this might work in practise.