`SerializationContext` missing from docs

[braingram]

SerializationContext is mentioned in the documentation for converters:
https://asdf.readthedocs.io/en/stable/api/asdf.extension.Converter.html#asdf.extension.Converter.select_tag
and in the future plans:
https://asdf.readthedocs.io/en/latest/asdf/developer_overview.html#future-plans-for-serializationcontext

But it's methods and usage is undocumented.