Expand extension point descriptions

[infotexture]

A topic with a list of all extension points is now generated automatically when the documentation build process runs.

The extension point descriptions are generated from the name attribute of each <extension-point> element in the plugin.xml configuration file for each plugin.

The conref mechanism should be used to expand or override the descriptions for the default toolkit plugins as used for the generated parameter listings and messages topics.

[infotexture]

The extension point topic generation mechanism was revised in d06e42f to use the separate topics that are now created for each plug-in.

Per discussion in the last docs call, the shared descriptions should be pushed to these topics rather than to the topic that lists all extension points.

Completed for HTML5 & XHTML in recent commits:

• HTML5: 4fa4bd6 (fixed Explain HTML5 parameter extension points #104)
• XHTML: 8b9da4d