Improve documentation on simulateDataStall

Explain that:
 - The method does not cause an actual data stall.
 - The detectionMethod parameter refers to
   ConnectivityDiagnosticsManager.DataStallReport constants (which
   already use an IntDef).
 - timestampMillis is relative to SystemClock.elapsedRealtime.

Fixes: 183972768
Test: m
Change-Id: I2eeb79e0e8ec6e8fd3b8fe1dcb1abbf2e6338983