doc: Documented how to set notification priority.

This is related to change http://ag/459520, where I added info about appropriate
priority constants for setPriority(int)

Doc is staged to:

http://asolovay.mtv:8998/guide/topics/ui/notifiers/notifications.html#Priority

Bug: 13514649
Change-Id: I792e2f02614c88fbcfe1b0714a9c10da27eb6c37