powerpc: Document details on H_HTM hcall

Add documentation to 'papr_hcalls.rst' describing the
input, output and return values of the H_HTM hcall as
per the internal specification.

Signed-off-by: Abhishek Dubey <adubey@linux.ibm.com>
Co-developed-by: Madhavan Srinivasan <maddy@linux.ibm.com>
Reviewed-by: Athira Rajeev <atrajeev@linux.vnet.ibm.com>
Signed-off-by: Madhavan Srinivasan <maddy@linux.ibm.com>
Link: https://patch.msgid.link/20250113164039.302017-3-adubey@linux.ibm.com