Docs: Clarify Matchers page to speak about Universal matchers

[clintoncwolfe]

Three small changes to the Matchers website page:

• Indicate that it refers to "universal" matchers (a term I just made up). Just trying to clarify that you can use these on any resource, while many resources also provide specialized matchers.
• Linkify the table of contents
• Add a note that RSpec offers matchers, too, but we don't support them. This probably needs some massaging.

[kagarmoe]

Much needed--Thanks! I pushed my changes to your branch.

[jquick]

Thanks @clintoncwolfe !