Add to docs comparison of tag vs $ref usage in schema.

[braingram]

The tag validator is mentioned in:
https://asdf.readthedocs.io/en/latest/asdf/extending/schemas.html#anatomy-of-a-schema

I am failing to find a place in the docs that mention:

• wildcard
• comparison of tag vs $ref for schema authors

These would be useful additions to the docs (possibly in the Extensions or Anatomy of a Schema section).