Document ACTION_WEBVIEW_SETTINGS possible unavailability

In some situations the activity start Intent
Settings#ACTION_WEBVIEW_SETTINGS might not resolve to any launchable
activity.  Make sure this is reflected in its reference documentation.

Bug: 157267574
Test: atest CtsSystemIntentTestCases
Test: atest CtsSystemIntentTestCases --user-type secondary_user
Change-Id: I59289cab764d06cb74e35722bd26986a3159e5d6

core/java/android/provider/Settings.java

@@ -1962,6 +1962,10 @@ public final class Settings {
      * Input: Nothing.
      * <p>
      * Output: Nothing.
+     * <p class="note">
+     * In some cases, a matching Activity may not exist, so ensure you
+     * safeguard against this.
+
      */
     @SdkConstant(SdkConstantType.ACTIVITY_INTENT_ACTION)
     public static final String ACTION_WEBVIEW_SETTINGS = "android.settings.WEBVIEW_SETTINGS";