Add descriptions to API schema

[rcannood]

Description of feature

When I make a call to Get schema, I get something like this:

core:
  run:
    fields_metadata:
      id:
        type: BigAutoField
        through: ~
        field_name: id
        model_name: run
        column_name: id
        schema_name: core
        is_link_table: no
        relation_type: ~
        related_field_name: ~
        related_model_name: ~
        related_schema_name: ~
      uid:
        type: CharField
        through: ~
        field_name: uid
        model_name: run
        column_name: uid
        schema_name: core
        is_link_table: no
        relation_type: ~
        related_field_name: ~
        related_model_name: ~
        related_schema_name: ~
      parent:
        type: ForeignKey
        through:
          left_key: parent_id
          right_key: id
          link_table_name: ~
        field_name: parent
        model_name: run
        column_name: parent_id
        schema_name: core
        is_link_table: no
        relation_type: many-to-one
        related_field_name: children
        related_model_name: run
        related_schema_name: core
...

Could we include documentation for modules, models, and fields? I need to be able to generate the R API documentation from this schema such that it looks something like this.