Replace "medication statement" to "medication usage".

In the description of MedicationAdministration there is a reference to the old term, which may raise doubts for the reader.

"This is a record of a medication being taken by a patient or that a medication has been given to a patient, where the record is the result of a report from the patient or another clinician. A medication statement is not a part of the prescribe->dispense->administer sequence but is a report that such a sequence (or at least a part of it) did take place, resulting in a belief that the patient has received a particular medication."