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

[積丹尼 Dan Jacobson]

This all has to be at the knowledge level of the user who has put his
first setq in his first .emacs file.

He uses describe-variable and is told some 3rd party has changed the
variable too and wants to know who.

PS> I'm somewhat inclined to suggest that IF something like this was
PS> done, a global switch would make more sense.

Indeed, the describe-variable docstring could mention: "if you want more
details, set "global-variable-tracking" to t, and restart
emacs. But note it will slow down emacs, so only use for debugging."

PS> It still might not have the effect you wanted, though -- it's
PS> possible to change the apparent / user-facing value of some variables
PS> without changing the *actual* value of the variable at all.  This is
PS> because of the internal structure of lists in lisp...

Sounds like a security / coverup risk. Maybe with the expensive
global-variable-tracking turned on, describe-variable could double check
for such tampering upon looking up a variable.