clarify that the CURLMOPT_TIMERFUNCTION callback can pass in 0 and -1 as legal

one of the \&"performing" functions of the multi interface

(\fIcurl_multi_socket(3)\fP, \fIcurl_multi_socket_all(3)\fP and

\fIcurl_multi_perform(3)\fP) - to allow libcurl to keep timeouts and retries

etc to work. Libcurl attempts to limit calling this only when the fixed future

timeout time actually change. See also \fICURLMOPT_TIMERDATA\fP. This callback

can be used instead of, or in addition to, \fIcurl_multi_timeout(3)\fP. (Added

in 7.16.0)

etc to work. A timeout value of -1 means that there is no timeout at all, and

0 means that the timeout is already reached. Libcurl attempts to limit calling

this only when the fixed future timeout time actually change. See also

\fICURLMOPT_TIMERDATA\fP. This callback can be used instead of, or in addition

to, \fIcurl_multi_timeout(3)\fP. (Added in 7.16.0)

.IP CURLMOPT_TIMERDATA

Pass a pointer to whatever you want passed to the

\fBcurl_multi_timer_callback\fP's third argument, the userp pointer.  This is