[alias] spec overview before alias-server implementation

Summary:
T3060

Following review of D13916, practical to keep any spec modifications to their own diff, and good idea to do this before they are actually implemented.

Test Plan: Review changes for consistency with desired approach.

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D13994