Document username restrictions

MB-22827 added a username length restriction to spock.

Usernames should be 64 utf8 characters or shorter, and may not be more than 128 characters.

This applies to both cluster administrator usernames and rbac usernames.

This should be documented on the "Initialize the Cluster" page, the Security > Security Best Practices > Couchbase Passwords page, and possibly in the CLI user-manage command.