Update documentation for entity reference to clarify CURIE/URI type (#…

…358)

Resolves mapping-commons/sssom-py#512

- [X] `docs/` have been added/updated if necessary

The documentation for "entity references" is largely confusing, and
leading, in its current form, to the expectation that metadata fields
types with these can be _either_ a CURIE _or_ a full URI.

This is not quite right: in the TSV serialistion, as well as JSON, we
expect the values to be CURIE-strings. This PR documents this design
decision, but, as @joeflack4 puts it rightfully, this has a bit of a
"smell" to it. Not sure what the right thing is here, but this
documentation here is certainly better than not having it.

src/sssom_schema/schema/sssom_schema.yaml

@@ -64,9 +64,12 @@ enums:
 types:
  EntityReference:
     typeof: uriorcurie
-    description: A reference to a mapped entity. This is represented internally as a string, and as a resource in RDF
+    description: |
+      A reference to an entity involved in the mapping.
     base: str
     uri: rdfs:Resource
+    see_also:
+      - https://mapping-commons.github.io/sssom/spec/#tsv
 
 slots:
   mirror_from: