docs: Update to doc on listening for restrictions-changed intent

Per bug 19848206, it turns out that APPLICATION_RESTRICTIONS_CHANGED
is only sent to dynamically-declared listeners, *not* to listeners
declared in the manifest. Updated doc to note this. (The code snippet
already showed how to dynamically declare a listener.)

See first comment for doc stage location.

bug: 19848206
Change-Id: I19c7150a9f36de226d00ad6d8db0e9210d5d135d