Updated documentation for OnSharedPreferenceChangeListener.

Clarified that #onSharedPreferenceChanged does not get called after
Editor#clear.

Bug: 117752822
Test: atest cts/tests/tests/content/src/android/content/cts/SharedPreferencesTest.java#testSharedPrefsChangeListenerIsCalledOnCommit
Test: atest cts/tests/tests/content/src/android/content/cts/SharedPreferencesTest.java#testSharedPrefsChangeListenerIsCalledOnApply
Change-Id: I6fd33d13ae706882131dafdfe0d095f9b48a1744