bug#14926: 24.3.50; Warning "`make-variable-buffer-local' should be called at toplevel"

[Drew Adams <drew.adams@oracle.com>]

> > 2. What is wrong with code such as this at top level?
> 
> Nothing.  But most of the cases I bumped into (all of which have
> been fixed now, AFAIK)

I guess you mean problems in the distributed code (?).

> were cases where the author had confused make-local-variable
> and make-variable-buffer-local.

Fair enough.  That is worth trying to help users with.  And it
has long been something needing reminding.

For things like that, I don't tend to favor compiler warnings.
I tend to favor improving the doc or adding a wiki page or a
tutorial video or...

Anyway, is the current approach (warning about anything not at
top level) the best way?  I think it confuses more than it helps.
I understand that it is well-intentioned.  But I don't think the
resulting noise and confusion are worth it.  If the lesson is
not clear and accurate then the course isn't worth taking or
giving.

Can you characterize better the case where we want people to use
(or not to use) `make-variable-buffer-local'?  IOW, just saying
you should use this at top level is too gross (and not correct).

Can you improve the spec a bit and then the implementation so
the warning appears only when it should (or almost only then)?