clarify

Author: MarnixCroes

docs/interface/02-send.md

@@ -27,6 +27,8 @@ addresses.
 
 !!! Warning
     Using the _Sweep_ button to empty a jar can significantly override the set fee limit.
+    This is because of the lack of change output.
+    The (default) maximum difference is 80%.
 
 After you made your first collaborative transaction with one of your jars as the
 recipient, you are ready to earn.