This expands upon the PMD docs, providing a list of file types that are

supported, as well as relevant file extensions. A handy list of URLs to

Java implementations for each file type in PMD (each containing the list

of file extensions) is provided to make future maintenance on this file

type list a bit easier.
Some wording in the Configuration section was also changed for

consistency.

Built the docs. Checked for build errors, spelling errors, and bad links.