Update API docs for TelecomManager#endCall.

Make it clear that ending an emergency call is not possible with this
method.

Test: Compile; this is a docs change.
Bug: 132438333
Change-Id: I736b90e31723ee2055cedf16142c4d0f494bf49b

telecomm/java/android/telecom/TelecomManager.java

@@ -1317,6 +1317,9 @@ public class TelecomManager {
      * foreground call is ended.
      * <p>
      * Requires permission {@link android.Manifest.permission#ANSWER_PHONE_CALLS}.
+     * <p>
+     * Note: this method CANNOT be used to end ongoing emergency calls and will return {@code false}
+     * if an attempt is made to end an emergency call.
      *
      * @return {@code true} if there is a call which will be rejected or terminated, {@code false}
      * otherwise.