armv8r64-extras/classes: xen_dom0less_config documentation improvements

Add additional documentation on the domain configuration variables and
their varflags.

Convert required_dom_varflags to a Bitbake variable.

Remove a reference to internal documentation.

Issue-Id: SCM-4387
Signed-off-by: Peter Hoyes <Peter.Hoyes@arm.com>
Change-Id: I5ba8300e6bba2c0934acf9cd4e5b4c59cf863493