xen: write a high level description of the sub-arch choices for heap layout

xen: write a high level description of the sub-arch choices for heap layout

The 3 options which (sub)arches have for the layout of their heaps is
a little subtle (in particular the two CONFIG_SEPARATE_XENHEAP=n
submodes) and can be a bit tricky to derive from the code.

Therefore try and write down some guidance on what the various modes
are.

Note that this is intended more as a high level overview rather than a
detailed guide to the full page allocator interfaces.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>