help/keybindings: document CutLine behavior

Author: dmaluka

runtime/help/keybindings.md

@@ -274,6 +274,14 @@ Autocomplete
 The `StartOfTextToggle` and `SelectToStartOfTextToggle` actions toggle between
 jumping to the start of the text (first) and start of the line.
 
+The `CutLine` action cuts the current line and adds it to the previously cut
+lines in the clipboard since the last paste (rather than just replaces the
+clipboard contents with this line). So you can cut multiple, not necessarily
+consecutive lines to the clipboard just by pressing `Ctrl-k` multiple times,
+without selecting them. If you want the more traditional behavior i.e. just
+rewrite the clipboard every time, you can use `CopyLine,DeleteLine` action
+instead of `CutLine`.
+
 You can also bind some mouse actions (these must be bound to mouse buttons)
 
 ```