memquota account architecture documentation

In !2493 (merged), @nickm observes that the documentation for the different Accounts, their "collapse structure", and so on, is rather thin. Also there are missing cross-references.

Probably,

• The information in mq_queue.rs under "Use in Arti" should move to tor_proto::memquota
• And be substantially expanded, and
• cross references added in relevant places (mq_queue itself at least).

Specific discussions/questions that the new descriptions/docs should answer:

• !2493 (comment 3089244)
• !2493 (comment 3089246)
• !2493 (comment 3089250)
• !2493 (comment 3089253)

(tickboxes ticked in anticipation of my TBD MR)

Part of #351 (closed)