[alias docs] Better language and implementation rule for possible price changes

Summary:
Best case scenario, alias pricing never changes. However, we want price changes to be possible. We also want to ensure that valid alias registrations are not invalidated by a price change.

Hence, any price change will need to come into effect for alias registrations confirmed in a specified blockheight.

1000 blocks is arbitrarily chosen as a minimum notice period. This allows alias-server to provide a stateful price response that Cashtab may parse for assessing whether or not its registration txs are valid.

Test Plan: Review for typos, comment if 1000 blocks seems inappropriate, comment any questions or ambiguity about how this could work

Reviewers: #bitcoin_abc, emack, Fabien

Reviewed By: #bitcoin_abc, emack, Fabien

Subscribers: Fabien, emack

Differential Revision: https://reviews.bitcoinabc.org/D14267