update to in-thread notification description

I felt that the description of what "in-thread notification" could be difficult to understand for some. This is better, I think (though I'm not fully satisfied).

res/values/strings.xml

@@ -438,7 +438,7 @@
     <string name="preferences__select_led_color">Select LED Color</string>
     <string name="preferences__sound">Sound</string>
     <string name="preferences__inthread_notifications">In thread notifications</string>
-    <string name="preferences__play_inthread_notifications">Play notification sound while in thread.</string>
+    <string name="preferences__play_inthread_notifications">Play notification sound when viewing an active conversation.</string>
     <string name="preferences__vibrate">Vibrate</string>
     <string name="preferences__also_vibrate_when_notified">Also vibrate when notified</string>
     <string name="preferences__minutes">minutes</string>