Update draft-ietf-cats-usecases-requirements.md

[bpatient78]

Update Introduction, Terminologies and part of requirements. 1)The introduction part has been rewritten to avoid some verbose description. 2)Recent update on the definition of service related terms has been added into the Terminology part, as well as the definition on "network edge" and "edge computing", in order to make clarification. 3)Requirements on "service instance selection" have been adjusted. 4)Some vague expressions like"quickly select" and "flexible use" have been modified or explained. 5)Requirement on "metric collection" has been added. 6)Requirements on "Session and service continuity" have been modified.

[VMatrix1900]

All changes are merged into the latest draft.