Say that C-d and DEL with arg do killing.

etc/TUTORIAL

@@ -384,7 +384,8 @@ Reinsertion of killed text is called "yanking".  Generally, the
 commands that can remove a lot of text kill the text (they set up so
 that you can yank the text), while the commands that remove just one
 character, or only remove blank lines and spaces, do deletion (so you
-cannot yank that text).
+cannot yank that text).  <Delback> and C-d  do deletion in the simplest
+case, with no argument.  When given an argument, they kill instead.
 
 >> Move the cursor to the  beginning of a line which is not empty.
    Then type C-k to kill the text on that line.