Standard inconsistency about tag attribute

[braingram]

The schema information docs mention:
https://asdf-standard.readthedocs.io/en/1.1.1/schemas.html#schema-implementation

The following top-level attributes are required for all ASDF schemas:

• $schema: Indicates the metaschema definition used to validate this schema
• id: A name that uniquely identifies the schema
• tag: The YAML tag corresponding to the type described by this schema

However, later on the same page
https://asdf-standard.readthedocs.io/en/1.1.1/schemas.html#tag
the tag attribute is described as optional:

While the id attribute will almost certainly become required in a future version of the ASDF Standard, the tag attribute may remain optional. This is because schemas can be referenced by id without necessarily referring to a particular tagged type in the YAML representation.

Given the work to remove the top level tag attribute (which limits schema re-use). I suggest we remove any description of the top-level tag attribute.