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