Clarify author notification requirements for auth48

[rjsparks]

Lets reconfirm what/when an author should receive email, focusing first on auth48. (We can open other issues to make sure we've covered email in other parts of the workflow).

Note https://github.com/rfc-editor/rpcwebsite/pull/70/files as input

cc: @ajeanmahoney

[ajeanmahoney]

Currently AUTH48 is launched with two email messages sent nearly simultaneously to authors, with stream-specific approvers, WG chairs, and document shepherd CCed.

• An auto-generated message (the current template is https://github.com/rfc-editor/rpcwebsite/blob/master/rfc-ed/Templates/AUTH48-Msgs/auth48_xml.txt) that is sent by the editor when the document is ready for author review.
• A list of questions from the RPC.