Improve documentation for subject and object categories (#257)

* Improve documentation for subject and object categories

Solves #256

* Update sssom_schema.yaml

src/sssom_schema/schema/sssom_schema.yaml

@@ -128,10 +128,17 @@ slots:
   subject_category:
     description: The conceptual category to which the subject belongs to. This can
       be a string denoting the category or a term from a controlled vocabulary.
+      This slot is deliberately underspecified. Conceptual categories can range from
+      those that are found in general upper ontologies such as BFO (e.g. process, temporal region, etc) to those that serve
+      as upper ontologies in specific domains, such as COB or BioLink (e.g. gene, disease, chemical entity). The purpose of this
+      optional field is documentation for human reviewers - when a category is known
+      and documented clearly, the cost of interpreting and evaluating the mapping decreases.
     range: string
     examples:
       - value: UBERON:0001062
         description: (The CURIE of the Uberon term for "anatomical entity".)
+      - value: anatomical entity
+        description: (A string, rather than ID, describing the "anatomical entity" category. This is possible, but less preferred than using an ID.)
       - value: biolink:Gene
         description: (The CURIE of the biolink class for genes.)
   subject_type:
@@ -208,10 +215,17 @@ slots:
   object_category:
     description: The conceptual category to which the subject belongs to. This can
       be a string denoting the category or a term from a controlled vocabulary.
+      This slot is deliberately underspecified. Conceptual categories can range from
+      those that are found in general upper ontologies such as BFO (e.g. process, temporal region, etc) to those that serve
+      as upper ontologies in specific domains, such as COB or BioLink (e.g. gene, disease, chemical entity). The purpose of this
+      optional field is documentation for human reviewers - when a category is known
+      and documented clearly, the cost of interpreting and evaluating the mapping decreases.
     range: string
     examples:
       - value: UBERON:0001062
         description: (The CURIE of the Uberon term for "anatomical entity".)
+      - value: anatomical entity
+        description: (A string, rather than ID, describing the "anatomical entity" category. This is possible, but less preferred than using an ID.)
       - value: biolink:Gene
         description: (The CURIE of the biolink class for genes.)
   mapping_justification: