[alias spec] Remove info about non-alphanumeric aliases

Summary:
Also specify that they're utf-8 encoded.
In effect this makes the characters all 1 byte ASCII equivalent, due to the aplphanumeric restriction.

Test Plan: Read to see that it makes sense

Reviewers: bytesofman, emack, #bitcoin_abc

Reviewed By: emack, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D13736