Add descriptions of subsets

[allenbaron]

It would be good to provide a description of each subset so that users can better understand their design and purposes. It should be pretty straightforward to do this.

As examples, Uberon, MP, and CL all use rdfs:comment to describe subsets. DO can do the same. If a more official annotation property is created in OMO (information-artifact-ontology/ontology-metadata#80), we can always use that instead.

The use of rdfs:label is less common than using comments to describe the subset, but DO is not the only ontology including a label.

[allenbaron]

It might be good to establish a common pattern for these descriptions. They generally are either 1) created for a specific project/user or 2) created to include a related set of terms. For some subsets, it might make sense to include both of these elements in the description.

Example patterns:

• MP: "Terms in use by {user}"
• CL: "a subset of general classes related to {description of cells/cell grouping}"
  _Uberon does not use a consistent pattern; it has some comments with labels, acronyms, and descriptions of varying length.