Improve documentation for documentation module.

[carltongibson]

In preparing #5448 I noted that documentation.py is essentially undocumented. There is a basic usage example for include_docs_urls in the Documenting your API topic page, but that's it.

We need documentation for the three functions:

• include_docs_urls
• get_schemajs_view
• get_docs_view

This would help users wanting to customise the defaults here.

[carltongibson]

Duplicate of #4980