[AWARE] Update docs for service discovery callbacks

Clarify that service discovery callbacks may be triggered multiple
times (and that therefore apps should deal correctly with it). These
may happen for several reasons (not documented since too low level):
- Peer went out-of-hearing-range and came back
- Peer went out of geofence range and came back
- Concurrency limitations caused discovery to switch from ranging-based
  to non-ranging-based or back

Bug: 77925576
Test: N/A
Change-Id: I9fbb909e5bd57fb714aa82bcfeb2834361722149