bug#13823: 24.3.50; Elisp manual description of property `pure'

[Stefan Monnier]

>> If you can come up with a good description of what it does, I'm all ears.
> There is a nice description of pure in byte-opt.el
>     ;; pure functions are side-effect free functions whose values depend
>     ;; only on their arguments.  For these functions, calls with constant
>     ;; arguments can be evaluated at compile time.  This may shift run
>     ;; time errors to compile time.

Sounds OK.  Feel free to put it in.


        Stefan