Mention potentially storing API response in section 2.1

[Kyle Larose]

Roman writes,

** Section 4.2.  Between step #2 and #3, did some kind of signaling happen to
indicate that expiration is imminent, or did the UE keep state of some kind?
Keeping state isn’t mentioned as a UE requirement in Section 2.1.  Section 2.5.
notes that a “signal might be generated when the end of a session is imminent”.

The implication is that there is state in this case -- the UE stored the last response
it retrieved from the API so that it could consult the time remaining (seconds-remainingin the API document). I can update section 2.1 to indicate that the UE could store informationfrom the API's response in order to trigger workflows related to imminent expiry of its access.Storing the state isn't required, but it could make the user experience better if supported.