[ARIA] updated text for rules on extensions to ARIA 1.x

As per our discussion today, I did make some minor modifications to the
text in the Wiki [1].  Cynthia was going to look and ensure that the
guidelines would permit extensions to things other than the roles, states,
and properties.  But this is basically the structure for creating
extensions that was tentatively agreed by the ARIA meeting.

[1] https://www.w3.org/WAI/PF/wiki/ARIAExtensions

-- 
Shane McCarron
Managing Director, Applied Testing and Technology, Inc.