bug#67455: Record source position, etc., in doc strings, and use this in *Help* and backtraces.

[Stefan Monnier via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

>> Why include the file name?  Presumably we will rely on a `(FILE . POS)`
>> to find that docstring so including FILE in there is rather redundant
>> (and in addition to that, this file name can be wrong if it's absolute
>> since files often get moved between compilation and use).
> No, the (FILE . POS) is the .elc file, the one I'm putting into the new
> info is the source file.

Until now, Emacs has always lived with the `.elc` file names (e.g. in
`load-history`) and managed to find the corresponding `.el` file (when
you try to jump to the source via `M-.` or by clicking in `C-h f`).
It comes with its share of problems, but we've learned to live with them.

Is there any reason you think this new functionality should go through
the extra trouble to record the `.el` name (and thus develop its own
set of workarounds for the cases where the `.el` doesn't live where we
think it should)?

>> > the position of the defining symbol in the file, and the position of
>> > the lambda (should we be in one) in the file.
>> Why two positions?
>> Why not use the same position info as used in `byte-compile-warn-x`?
> I'm not sure, yet, but I suspect both positions will be useful.

Can you give an example scenario?

>> > or (defining-symbol (if (consp struct) (car struct) struct))
>> [ Haven't seen that yet.  ]
> Have a look at cl-defgeneric and cl-defmethod in cl-generic.el.

Ah, got it.  I like the way you can refer to args by name.
So I guess all the (defining-symbol 1) could similarly be replaced with
(defining-symbol THE-ARG-NAME).

[ I also notice that this extension is somewhat incompatible with the
  use I proposed for `font-lock`.  :-(  ]

[ As you noted, this info is also related to the `&define` debug spec.
  I wish we could unify that information.  ]


        Stefan