MM documentation: add "Unaccepted" meminfo entry

Commit dcdfdd40 ("mm: Add support for unaccepted memory") added a
entry to meminfo but did not document it in the proc.rst file.

This counter tracks the amount of "Unaccepted" guest memory for some
Virtual Machine platforms, such as Intel TDX or AMD SEV-SNP.

Add the missing entry in the documentation.

Link: https://lkml.kernel.org/r/20250317230403.79632-1-npache@redhat.com


Signed-off-by: Nico Pache <npache@redhat.com>
Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Acked-by: David Hildenbrand <david@redhat.com>
Cc: Andrii Nakryiko <andrii@kernel.org>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Jeff Xu <jeffxu@chromium.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Pasha Tatashin <pasha.tatashin@soleen.com>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: xu xin <xu.xin16@zte.com.cn>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>