MM documentation: add "Unaccepted" meminfo entry

Commit dcdfdd4 ("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>

Author: PacheNico

Documentation/filesystems/proc.rst

@@ -1081,6 +1081,7 @@ Example output. You may not have all of these fields.
     FilePmdMapped:         0 kB
     CmaTotal:              0 kB
     CmaFree:               0 kB
+    Unaccepted:            0 kB
     Balloon:               0 kB
     HugePages_Total:       0
     HugePages_Free:        0
@@ -1256,6 +1257,8 @@ CmaTotal
               Memory reserved for the Contiguous Memory Allocator (CMA)
 CmaFree
               Free remaining memory in the CMA reserves
+Unaccepted
+              Memory that has not been accepted by the guest
 Balloon
               Memory returned to Host by VM Balloon Drivers
 HugePages_Total, HugePages_Free, HugePages_Rsvd, HugePages_Surp, Hugepagesize, Hugetlb