Meaning of arg in preedit-text events on pgtk

[Po Lu]

I documented the `preedit-text' event (which is now used by both the
PGTK port and the X port), but I don't know the meaning of `arg' under
PGTK.

Yuuki, could you please write that part of the documentation?  It's
under the node "Misc Events" in doc/lispref/commands.texi.  Thanks.

Also, does this event need a NEWS entry?  It was originally specific to
the PGTK port, but after implementing the preedit text feature on X, it
might need a NEWS entry now.

Thanks.