Clarify which email addresses may be set as primary

[janiceilene]

Issue overview

This relates to the Changing your primary email address article. A user found that they could not change their primary email address to an email that is already set to be their backup email address.

Content strategy plan

• Add some text before the procedure that you cannot choose your backup email address as your primary.
• To work around this, we recommend setting backup to "allow all verified emails"