pgtable: improve pte_protnone() comment

Especially the "For PROT_NONE VMAs, the PTEs are not marked
_PAGE_PROTNONE" part is wrong: doing an mprotect(PROT_NONE) will end up
marking all PTEs on x86_64 as _PAGE_PROTNONE, making pte_protnone()
indicate "yes".

So let's improve the comment, so it's easier to grasp which semantics
pte_protnone() actually has.

Link: https://lkml.kernel.org/r/20230803143208.383663-6-david@redhat.com
Signed-off-by: David Hildenbrand <david@redhat.com>
Acked-by: Mel Gorman <mgorman@techsingularity.net>
Cc: Hugh Dickins <hughd@google.com>
Cc: Jason Gunthorpe <jgg@ziepe.ca>
Cc: John Hubbard <jhubbard@nvidia.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: liubo <liubo254@huawei.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Peter Xu <peterx@redhat.com>
Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>