-The copy starts out with the same definitions of KEYMAP,
-but changing either the copy or KEYMAP does not affect the other.
+
+Note that this is almost never needed. If you want a keymap that's like
+another yet with a few changes, you should use map inheritance rather
+than copying. I.e. something like:
+
+ (let ((map (make-sparse-keymap)))
+ (set-keymap-parent map <theirmap>)
+ (define-key map ...)
+ ...)
+
+After performing `copy-keymap', the copy starts out with the same definitions
+of KEYMAP, but changing either the copy or KEYMAP does not affect the other.