Update "Change Username" instructions to be less foreboding

Summary:
Ref T13420. These warnings are currently more severe than they need to be; weaken them.

Among other cases, the upstream supports and encourages changing usernames when users change human names.

The login/password instructions are also out of date since sessions were decoupled from usernames about a year ago.

Test Plan: Hit dialog as an administrator

Maniphest Tasks: T13420

Differential Revision: https://secure.phabricator.com/D20826