news and docs for command-error-function

[Jarek Czekalski <jarekczek@poczta.onet.pl>]

Stefan,

We modified keyboard.c (r114987 114988 114991) and in 114988 you removed 
the following from the docs:

-This variable is initialized with Emacs default error output function.
-It is suggested that user functions are added using add-function. */);

I thought it would be good to describe 3 things:

1. suggested way to use this variable (add-function)
2. the change in its handling introduced in 24.4
3. the way it could be substituted before 24.4 (a lisp snippet, like in 
http://lists.gnu.org/archive/html/emacs-devel/2013-10/msg00819.html)

Now I also think both the variable and the function should go to NEWS.

I can prepare these changes if you like, but first I wanted to have an 
agreement of what should be present there. If someone else wants to do 
the docs themselves, no problem.

Jarek