update wording w/r to link relations tied to descriptors

FOR DISCUSSION ONLY. DO NOT MERGE.
A representation that specifies link relation types, may use any of the following values:

1. A registered IANA link relation type (e.g. rel="edit").

2. An extension link relation type as defined by <xref target="RFC5988"/> whose value is
the unqualified URI of an associated descriptor (e.g. rel="http://alps.io/profiles/item#purchased-by").

3. The 'id' or 'name' of an associated state transition descriptor property (e.g. rel="purchased-by") 
per the conventions of section 2.2.7.1 and section 2.2.7.3 if the representation includes an ALPS profile.