Clarify ability to use @MessageMapping methods on both @Controller as
well as @RestController.
Add section on configuring connections (including credentials) to the
message broker and clarify the use of the login/passcode headerers of
the STOMP CONNECT frame.
Add note on when to add the reactor-tcp dependency.
Issue: SPR-11464, SPR-11436, SPR-11449
651e0a44fb