mm: document FAULT_FLAG_VMA_LOCK flag

mm: document FAULT_FLAG_VMA_LOCK flag

FAULT_FLAG_VMA_LOCK flag was introduced without proper description.  Fix
this by documenting it.

Link: https://lkml.kernel.org/r/20230301022720.1380780-2-surenb@google.com
Fixes: 863be34fc093 ("mm: add FAULT_FLAG_VMA_LOCK flag")
Signed-off-by: Suren Baghdasaryan <surenb@google.com>
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Link: https://lore.kernel.org/all/20230301113648.7c279865@canb.auug.org.au/
Cc: Dan Carpenter <error27@gmail.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: David Howells <dhowells@redhat.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Jason Gunthorpe <jgg@ziepe.ca>
Cc: Laurent Dufour <laurent.dufour@fr.ibm.com>
Cc: Liam R. Howlett <Liam.Howlett@oracle.com>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Pavel Tatashin <pasha.tatashin@soleen.com>
Cc: Yu Zhao <yuzhao@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>