bug#7751: 24.0.50; `fill-paragraph' on doc string with colons

[Drew Adams]

> the colon is there so that forms like
> (defcustom foo nil
>   :bar 3)
> 
> Don't get refilled to
> (defcustom foo nil :bar 3)
> 
> when you type M-q.

And just why is that important?

If there is room for it given the value of `fill-column', such filling makes
sense - a good thing, not a bad thing.

Sure, it would be best if a keyword and its value were always (`fill-column'
permitting) on the same line.  But that's a nice-to-have - it should not be the
first priority or the reason for mangling filling of doc strings.

Anyway, the #7751 bug report is not about that, at all.

It's about using `M-q' on a _doc string_.  It's not about using `M-q' on a sexp
such as you present above (which has no strings at all).  Let's not take a
detour.  If the current code does not distinguish the doc-string case, maybe it
should.