KVM: TDX: Document alignment requirements for KVM_TDX_INIT_MEM_REGION

KVM: TDX: Document alignment requirements for KVM_TDX_INIT_MEM_REGION

Since it was never possible to use a non-PAGE_SIZE-aligned @source_addr,
go ahead and document this as a requirement. This is in preparation for
enforcing page-aligned @source_addr for all architectures in
guest_memfd.

Reviewed-by: Vishal Annapurve <vannapurve@google.com>
Tested-by: Kai Huang <kai.huang@intel.com>
Signed-off-by: Michael Roth <michael.roth@amd.com>
Reviewed-by: Yan Zhao <yan.y.zhao@intel.com>
Link: https://patch.msgid.link/20260108214622.1084057-6-michael.roth@amd.com
Signed-off-by: Sean Christopherson <seanjc@google.com>