bug#39802: (info "(emacs) Init Rebinding") needs to say it is impossible to unbind local keys

[Eli Zaretskii]

> From: Noam Postavsky <npostavs@gmail.com>
> Date: Thu, 27 Feb 2020 08:14:56 -0500
> Cc: 39802@debbugs.gnu.org
> 
> +@findex define-key
>    As described in @ref{Local Keymaps}, major modes and minor modes can
> -define local keymaps.  These keymaps are constructed when the mode is
> -used for the first time in a session.  If you wish to change one of
> -these keymaps, you must use the @dfn{mode hook} (@pxref{Hooks}).
> +define local keymaps.  The function @code{define-key} can be used to
> +make changes in a specific keymap.  This function can also unset keys,
> +when passed @code{nil} as the binding.

This loses useful information in the sentence that starts with "These
keymaps".

Otherwise I'm okay with the proposed change.