libcurl documentation: clarifications and typos

* Elaborates on default values of some curl_easy_setopt() options.
* Reminds the user to cast variadic arguments to curl_easy_setopt() to
  'void *' where curl internally interprets them as such.
* Clarifies the working of the CURLOPT_SEEKFUNCTION option for
  curl_easy_setopt().
* Fixes typo 'forth' → 'fourth'.
* Elaborates on CURL_SOCKET_TIMEOUT.
* Adds some missing periods.
* Notes that the return value of curl_version() must not be passed to
  free().

docs/libcurl/curl_multi_cleanup.3

@@ -41,6 +41,7 @@ handle is no longer connected to the multi handle
 3 - \fIcurl_multi_cleanup(3)\fP should be called when all easy handles are
 removed
 .SH RETURN VALUE
-CURLMcode type, general libcurl multi interface error code.
+CURLMcode type, general libcurl multi interface error code. On success,
+CURLM_OK is returned.
 .SH "SEE ALSO"
 .BR curl_multi_init "(3)," curl_easy_cleanup "(3)," curl_easy_init "(3)"