[v3] Document iword, pword, callback events

[Benjamin Kosnik]

>2003-07-09  Jerry Quinn  <jlquinn@optonline.net>
>
>	* include/bits/basic_ios.h (copyfmt): Document.
>	* include/bits/ios_base.h (event, event_callback, register_callback,
>	xalloc, iword, pword):  Document.
>	(imbue, ~ios_base): Update docs on callbacks.

Thanks very much for doing this. 

-benjamin