[DOC] Regexp term clarification

The `RegExp` query didn't behave like I expected, and only after some trial-and-error and searching StackOverflow did I finally realize that "term queries" was a very important part of the first sentence in the `RegExp` documentation.

This attempts to clarify that so that others do not suffer the same fate.

It also, incidentally, updates the Lucene javadoc link, which I just fixed while I was editing this file.