Documents the use of KillChainPhaseReferenceType

Specifically, notes the use of the redundant attributes should either be
omitted or, if present, should match the value in the definition.
Addresses #41.
Clarified @id, @phase_id, @kill_chain_id usage

Fixes #40
Merge pull request #115 from STIXProject/issue_41

Document the use of KillChainPhaseReferenceType