Document API change for GPGME_EVENT_DONE from 2009.

* doc/gpgme.texi (I/O Callback Interface): Fix description for the
event arg.

With commit c8e934b2 (2009-10-26) gpgme_io_event_done_data_t was
introduced to replace the use of gpg_error_t with GPGME_EVENT_DONE.
Unfortunately this was not documented. Maybe at that time the event
code was considered internal and its use in GPA was not known. Too
bad.