prctl.2, proc.5: Document VMA naming

prctl.2, proc.5: Document VMA naming

Bring in man page changes from the patch set by Colin Cross, and
document the corresponding virtual names in procfs.

Link: <https://lore.kernel.org/linux-mm/20211019215511.3771969-2-surenb@google.com/>
Signed-off-by: Nick Gregory <nick@nickgregory.me>
Reviewed-by: Suren Baghdasaryan <surenb@google.com>
Cc: Colin Cross <ccross@google.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Pasha Tatashin <tatashin@google.com>
Signed-off-by: Alejandro Colomar <alx@kernel.org>