Remove section 3 "The CAA RR Type" (#5)

Section 3 "The CAA RR Type" duplicated a lot of definitions from Section 5 "Mechanism." Both
sections contained normative references, so it was not always clear which version to refer to,
and the definitions were sometimes inconsistent.

This PR merges the two sections. In most cases, this was a no-op due to duplication. "The
CAA RR Type" had two "MUSTs" and one "MUST NOT." I believe I have preserved those
semantics.

This PR also moves "Use of DNS Security" under "Security Considerations."

This PR also tidies up some definitions under "Mechanism" that use "property", "value", and
"parameter" in their informal sense, which was confusing because these terms also have
specific technical meaning in the CAA spec.