Updating dox for mod_rest. Ideas expressed / clarified: 1) Making clear that mod_rest isn't to be installed under VirtualHosts AND as a component. 2) Understanding some of the implications of this choice: A) Changes to user authentication B) How it affects subdomains 3) More consistent use of domain names for clarity. 4) Using different heading sizes to show scope of section. Essentially, I added all the tidbits I had to clarify in getting this to work in my own example.

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summary: STARTTLS failure test
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This module responds to `<starttls xmlns='urn:ietf:params:xml:ns:xmpp-tls'/>` with
`<failure xmlns='urn:ietf:params:xml:ns:xmpp-tls'/>`, in order to test
how clients and server behave.

See [RFC6120 Section 5.4.2.2](https://xmpp.org/rfcs/rfc6120.html#rfc.section.5.4.2.2)