Update introduction of IANA Considerations
Add info about profile URIs to sections defining the resp. document form

/cc @msporny @gkellogg
Rename section Compact Document Form to Compacted Document Form

This makes the naming consistent with the other two document forms and the profile URI.
Fix bug in Create Term Definition algorithm

... which resulted in a cyclic IRI mapping error when a term was redefined to itself

See https://github.com/json-ld/json-ld.org/commit/657c90f3fc4050fce006fa11a2a420e7e4efe049

/cc @dlongley @gkellogg