Add docs for setSignalStrengthReportingCriteria

Because setSignalStrengthReportingCriteria only
supports a single measurement quantity, provide
further clarification on the applicability of the
API and how it may be used in various situations.

Bug: 110121199
Test: compilation - docstring-only change
Change-Id: If4236070fbc03368e5a78b0cf502cdc4a529a6ed