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>

tcl/interface/parport.cfg

@@ -1,6 +1,8 @@
 #
 # Parallel port wiggler (many clones available) on port 0xc8b8
 #
+# REVISIT this address seems very wrong.
+# Surely 0x378/LPT1 or 0x278/LPT2 ...
 
 interface parport
 parport_port 0xc8b8