Is there any easy way to determine the purpose of a prefix key shown in
the output of `(describe-key Chk`?

I walked through my `(kbd "C-x <key>")` and `(kbd "C-x C-<key>")` combinations 
to find out what they are all bound to.

The method worked fine, except for prefix keys. Then, the Chk output in the minibuffer
just showed the prefix keys typed so far, without saying something like "Prefix key for
xxx mode", or "Prefix key for register actions (C-x r)."

I looked at the output of `describe-prefix-bindings` too, but for prefix keys, it just
said things like this, without saying anything about the source or purpose of the prefix.

```lisp
C-x RET         Prefix Command
C-x ESC         Prefix Command
C-x 8           Prefix Command
C-x r           Prefix Command
C-x n           Prefix Command
C-x             Prefix Command
C-h 4           Prefix Command

```

How can I find out the purpose of a Prefix Command listed in the output of `describe-bindings`?