Fix incorrect new-owner prerequisite in ownership transfer

[stephanoodle]

A customer flagged that the "Before you start" section of Transfer Account Ownership told them the new owner "must have (or create) their own DNSimple user." That is backwards, and following it walks straight into an error.

Ownership is transferred by changing the user email on the account owner. UserChangeEmailInteractor rejects the change if any user identity already exists for the target email, returning "Email has already been taken" (interactor.message_error_user_emailtaken). User also validates email uniqueness. So the new owner must not already have a DNSimple user at that address.

Changes:

• Rewrote "Before you start" to explain that ownership is tied to the user email, that the new owner must not already have a DNSimple user at that address, and to name the exact error they will hit.
• Added the three ways out: use a different address, change the email on their existing user to free the address, or delete that user (with the data-loss caveat).
• Retitled the steps heading from "To transfer account ownership to another DNSimple user" to "...to another person," since the whole point is that they are not a separate user.

No cross-link changes needed. The two articles that link here describe it accurately.