doc: clarify that C and <a-C> act column-wise

Author: dontlaugh

README.asciidoc

@@ -632,7 +632,7 @@ use `n` followed by `,`, in order to remove a selection, use `<a-,>`.
 contains a match for this regex. Using `<a-K>` you can keep the selections
 not containing a match.
 
-`C` duplicates selections on the lines that follow them.
+`C` duplicates selections on the lines that follow them, column-wise.
 `<a-C>` does the same but on the preceding lines.
 
 `$` allows you to enter a shell command and pipe each selection to it.