correct mosquitto.h return code documentation

We currently erroneously claim that mosquitto_lib_init cannot fail,
while it can fail on Windows, if WSAStartup fails in net__init. Correct
this.

Signed-off-by: Martin Kelly <mkelly@xevo.com>

lib/mosquitto.h

@@ -202,7 +202,8 @@ libmosq_EXPORT int mosquitto_lib_version(int *major, int *minor, int *revision);
  * This function is *not* thread safe.
  *
  * Returns:
- * 	MOSQ_ERR_SUCCESS - always
+ * 	MOSQ_ERR_SUCCESS - on success.
+ * 	MOSQ_ERR_UNKNOWN - on Windows, if sockets couldn't be initialized.
  *
  * See Also:
  * 	<mosquitto_lib_cleanup>, <mosquitto_lib_version>