Update @2.0::IGnss.hal setCallback() method documentation

The IGnss.hal setCallback() and cleanup() methods need to be updated
to clarify when the framework calls them so that that the GNSS engine
knows when to shut down for power savings.

This CL updates the @2.0::IGnss.hal setCallback() method documenation
only. The @1.1::IGnss.hal and @1.0::IGnss.hal setCallback() and cleanup()
method documentation is updated in another CL.

Bug: 124104175
Test: Existing tests pass
Change-Id: I6a2dd6f82becc0adef8b4b56fe83e7c004aefd7a