bug#29991: Document how user can find all obsolete items he has set

[Eli Zaretskii]

> From: 積丹尼 Dan Jacobson
>       <jidanni@jidanni.org>
> Date: Fri, 05 Jan 2018 05:14:34 +0800
> 
> However one's .emacs file might be chock full of items such as
> 
>     x-select-enable-clipboard is a variable defined in ‘select.el’.
>     Its value is nil
> 
>       This variable is an alias for ‘select-enable-clipboard’.
>       This variable is obsolete since 25.1;
>       use ‘select-enable-clipboard’ instead.
> 
> Therefore the manual should mention how to systematically weed out all
> obsolete items that one has set in one's .emacs file.
> 
> Or there just answer the question 'Have I set any items that are obsolete?'.

Compile your .emacs, and the byte-compiler will flag obsolete
variables.