vmbus: fix ABI documentation

Fixes to vmbus ABI document including:
	- make it clear that relid is numeric value in sub directory
	- clarify interrupt mask description
	- spelling fixes
	- document regions

Signed-off-by: Stephen Hemminger <sthemmin@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>