Add a note about selection drag being auto-released

kitty/config_data.py

@@ -160,6 +160,11 @@ def uniq(vals: Iterable[T]) -> List[T]:
 
 You can run kitty with the :option:`kitty --debug-input` command line option
 to see mouse events. See the builtin actions below to get a sense of what is possible.
+
+.. note::
+    Once a selection drag is started, releasing the button that started it will
+    automatically end it.
+
 '''),
     ],
     'shortcuts': [