[racket] Scribble change, especially affects `scribble/sigplan'

The `scribble/base' and `scribble/manual' styles already put space
between blocks, and so no extra space is needed. The SIGPLAN style, in
contrast, uses indentation instead of space between paragraphs.
(Actually, the SIGPLAN style has an option for using space between
paragraphs, and probably code shouldn't have extra spacing in that
mode.)

I forgot to check `scribble/jfp'. It looks like extra space would make
sense for that one, too --- analogous to the extra space for \quote{}.

At Wed, 30 Mar 2011 16:11:24 -0600, Robby Findler wrote:
> This sounds great! One small question: why only in sigplan conf and
> not in all --pdf generated racketblock's?
> 
> Robby
> 
> On Wed, Mar 30, 2011 at 3:31 PM, Matthew Flatt <mflatt at cs.utah.edu> wrote:
> > As of v5.1.0.5, Scribble renders `racketblock', `codeblock',
> > `interactions', etc., in a different way than before. Indentation
> > relative to surrounding text is now implemented by a nested-flow style,
> > instead of adding spaces to the start of each line. (Using spaces was a
> > hack from before sorting out various rendering problems with nested
> > code blocks.)
> >
> > For most Scribble documents, you shouldn't notice the difference, but
> > `scribble/sigplan' mode for Latex/PDF output is an exception. The
> > Scribble style adjustments for the SIGPLAN format now introduce
> > vertical space before and after a code block. The change is intended to
> > make uses of `racketblock', `codeblock', etc., look better by default,
> > but the change could make existing documents look worse.
> >
> > To disable the extra space (because you don't like it or because
> > you've already added space in some other way), add these commands in
> > your document's Latex configuration:
> >
> >  \renewcommand{\SCodePreSkip}{}
> >  \renewcommand{\SCodePostSkip}{}
> >
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