chore: document valid identity names

[sesi200]

Description

The dfx identity new help text and documentation don't state what a valid identity name looks like.

Fixes SDK-1481

How Has This Been Tested?

Not tested

Checklist:

• [x] The title of this PR complies with Conventional Commits.
• [x] I have edited the CHANGELOG accordingly.
• [x] I have made corresponding changes to the documentation.