net/ppp: fix comments - ppp_{sync,asynctty}_receive() may sleep

The receive_buf methods of the N_PPP and N_SYNC_PPP line disciplines,
ppp_asynctty_receive() and ppp_sync_receive(), call tty_unthrottle()
which may sleep. Fix the comments claiming otherwise.

Impact: documentation
Signed-off-by: Tilman Schmidt <tilman@imap.cc>
Signed-off-by: David S. Miller <davem@davemloft.net>