Clarify what may be returned user email search results

What does this MR do?

The docs state that searching on an email address will return an exact match. This is true, but in some instances it may also return a partial match. For example, if you search for the email oo@bar.org the user with that address will be returned, but a user with the email address foo@bar.org would also be returned.

This clarification would have prevented this ticket (internal use).

Related issues

#26970

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