Document user-driven merging of identities

[mooreds]

This is a use case that occurs where we want to allow users to merge their accounts. This is custom code, but would be a great example app.

We'd need to cover safe ways to merge accounts that would avoid account takeover (so showing proof of ownership) and not letting folks see other users.

One idea would be to let folks search for accounts associated with a phone number or email address, but only show the results after a confirmation code was sent to that phone number or email address and then entered by the user.