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

[積丹尼 Dan Jacobson]

(OK, it's like telling the user your current balance is $0, your original
balance is $1000, and throwing out even the final record of what happened.
At least don't tell him his original balance then.)

Gasp, that var-watcher stuff is so complicated.

How about:
At the beginning of the user's .emacs:
(setq tracked-variables LIST_OF_TRACKED_VARIABLES)

And then if he later does describe-variable on one of them,
he gets all the places and values that variable was set.

The tracked-variables docsrting should warn: this is for debugging use
only, and adds a burden...