bug#38173: describe-variable: Also tell user *where* variable was changed

[積丹尼 Dan Jacobson]

^H v w3m-command

   w3m-command is a variable defined in ‘w3m.el’.
   Its value is "/usr/bin/w3m"
   Original value was nil

User thinks:
Hmmm, it was changed I see.
Where did the change happen?
Did it get changed in w3m.el? But that's where it was first defined. So
it wouldn't have happened there.
He looks in .emacs . Wait, he doesn't even have a .emacs.
He gives up.

So you only tell him half the story.
Either record at least the last place a variable was changed.
Or don't tell him it was changed.

emacs-version "26.3" .

Yes, if he changed in by hand in his *scratch* buffer etc., even say that too.