implementation and usage of ancillary bus

[dmertman]

This patch is to update the Docuemtnation file for vitual
bus both my changing the name to ancillary, and also to add
some important clarification to the Documentation.

The clarification will cover the explicit memory model of the
ancillary bus devices, and when the ancillary bis should, and
should not, be used.

Change-Id: I0fc79fafb5d4998a07b9b37dab8bd8ce613acb33
Change-type: Other
Title: Changing Documentation from virtual bus to ancillary