[backport#16847] doc: add comments clarifying how local services are advertised

Summary:
PR description:

Recent questions have come up regarding dynamic service registration
(see https://github.com/bitcoin/bitcoin/pull/16442#discussion_r308702676
and the assumeutxo project

While investigating how dynamic service registration might work, I was
confused about how we convey local services to peers. This adds some
documentation that hopefully clarifies this process.

Backport of PR16847

Test Plan: ninja (code comments only)

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, jasonbcox

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D8010