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

[積丹尼 Dan Jacobson]

In the emacs manual (Info-index "obsolete command")
only finds

       When ‘M-x’ completes on commands, it ignores the commands that are
    declared “obsolete”; for these, you will have to type their full name.
    Obsolete commands are those for which newer, better alternatives exist,
    and which are slated for removal in some future Emacs release.

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      This variable is obsolete since 25.1;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?'.