Add some explaination to the layout of vmemmap virtual address
space and how physical page mapping is only used for valid PFNs
present at any point on the system.
Reviewed-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Signed-off-by: Anshuman Khandual <khandual@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>