bug#39630: Mention how to unset keys in Your Init File

[Eli Zaretskii]

> From: 積丹尼 Dan Jacobson
>  <jidanni@jidanni.org>
> Date: Sun, 16 Feb 2020 11:28:41 +0800
> 
> 49.3.5 Changing Key Bindings Interactively.
>        mentions how to set and unset keys.
> 49.3.6 Rebinding Keys in Your Init File
>        mentions how to set keys, but not how to unset keys.

I think there's a limit to what we can say in the user manual that
describes Lisp facilities.  Users who are serious about key-binding
customizations should read the ELisp manual to augment and co0mplement
what is in the user manual.