parport (mostly) doc fixes

parport (mostly) doc fixes

The "parport_port" commands generally don't *require* a port_number;
they're of the "apply any parameter, then print result" variety.  Update
the User's Guide accordingly.

Some of those commands are intended to be write-once: parport_port,
and parport_cable.  Say so.

Use proper EBNF for the parport_write_on_exit parameter.

Parport address 0xc8b8 is evidently mutant.  Say so in the "parport.cfg"
file, to avoid breaking anyone with that mutant config.  But update the
User's Guide to include a sane example for the LP2 port.

Finally document the "presto_serial" command.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>