bug#15116: 24.3.50; doc of `set-match-data'

[Drew Adams]

> Awesome, so now we can expect 100s of reports about this, complete
> with little lectures when we disagree.

Too bad.  It's not about minimizing development.  It's about helping
users and specifying clearly (yes, for developers too) just what the
language is.

If Common Lisp can do it (and has done it for 30 years) then the
self-vaunted "self-documenting" editor can do it too.

It just means thinking also `user' and `spec', not just `code'.
Emacs is a big boy now; he should be able to handle this.