powerpc: Document APIv2 KVM hcall spec for Hostwide counters

powerpc: Document APIv2 KVM hcall spec for Hostwide counters

Update kvm-nested APIv2 documentation to include five new
Guest-State-Elements to fetch the hostwide counters. These counters are
per L1-Lpar and indicate the amount of Heap/Page-table memory allocated,
available and Page-table memory reclaimed for all L2-Guests active
instances

Reviewed-by: Bagas Sanjaya <bagasdotme@gmail.com>
Tested-by: Gautam Menghani <gautam@linux.ibm.com>
Signed-off-by: Vaibhav Jain <vaibhav@linux.ibm.com>
Signed-off-by: Madhavan Srinivasan <maddy@linux.ibm.com>
Link: https://patch.msgid.link/20250416162740.93143-2-vaibhav@linux.ibm.com