bug#24014: 24.5; doc string of `cl-reduce'

[Drew Adams]

> >> Please state in the doc string what the function does.  IOW,
> >> explain/describe/specify what "reduce" means.  Do not suppose that Emacs
> >> Lisp users are all familiar with functional programming etc.
> 
> The documentation of `seq-reduce' is probably not perfect, but at least
> it tries to explain what reducing means:
> 
>     Return the result of calling FUNCTION with INITIAL-VALUE and the
>     first element of SEQUENCE, then calling FUNCTION with that result and
>     the second element of SEQUENCE, then with that result and the third
>     element of SEQUENCE, etc.
> 
>     If SEQUENCE is empty, return INITIAL-VALUE and FUNCTION is not called.

Much better, yes.  This is the right level at which to describe what
it does for most programmers, IMO.