docs: fix virEventAddHandle return details

In commit 6d41cb87, the interface for virEventAddHandleFunc was changed.
This patch updates the documentation for virEventAddHandle to reflect
the new significance of the return value. Also, both functions now
mention -1 for failure.

include/libvirt/libvirt.h.in

@@ -3643,8 +3643,8 @@ typedef void (*virEventHandleCallback)(int watch, int fd, int events, void *opaq
  * If 'ff' callbacks are invoked directly from the virEventRemoveHandleFunc
  * they will likely deadlock in libvirt.
  *
- * Returns a handle watch number to be used for updating
- * and unregistering for events
+ * Returns -1 if the file handle cannot be registered, otherwise a handle
+ * watch number to be used for updating and unregistering for events
  */
 typedef int (*virEventAddHandleFunc)(int fd, int event,
                                      virEventHandleCallback cb,

src/util/virevent.c

@@ -48,7 +48,8 @@ static virEventRemoveTimeoutFunc removeTimeoutImpl = NULL;
  *
  * Register a callback for monitoring file handle events.
  *
- * Returns -1 if the file handle cannot be registered, 0 upon success
+ * Returns -1 if the file handle cannot be registered, otherwise a handle
+ * watch number to be used for updating and unregistering for events
  */
 int
 virEventAddHandle(int fd,