libudev: clarify that udev_monitor_receive_device() is non-blocking

Based on a patch from: Sam Thursfield <ssssam@gmail.com>
https://bugs.freedesktop.org/show_bug.cgi?id=51202

Makefile.am

@@ -28,7 +28,7 @@ LIBUDEV_REVISION=1
 LIBUDEV_AGE=0
 
 LIBGUDEV_CURRENT=1
-LIBGUDEV_REVISION=1
+LIBGUDEV_REVISION=2
 LIBGUDEV_AGE=1
 
 LIBSYSTEMD_LOGIN_CURRENT=2

src/libudev/libudev-monitor.c

@@ -494,6 +494,11 @@ tag:
  *
  * Only socket connections with uid=0 are accepted.
  *
+ * The monitor socket is by default set to NONBLOCK. A variant of poll() on
+ * the file descriptor returned by udev_monitor_get_fd() should to be used to
+ * wake up when new devices arrive, or alternatively the file descriptor
+ * switched into blocking mode.
+ *
  * The initial refcount is 1, and needs to be decremented to
  * release the resources of the udev device.
  *