seccomp.2: Rework SECCOMP_GET_NOTIF_SIZES somewhat

The existing text says the structures (plural!) contain a 'struct
seccomp_data'. But this is only true for the received notification
structure (seccomp_notif). So, reword the sentence to be more
general, noting simply that the structures may evolve over time.

Add some comments to the structure definition.

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
1 file changed