scribble/comment-reader, unsyntax, and RACKETBLOCK

While writing a paper in scribble, I noticed scribble/comment-reader and RACKETBLOCK don't play nicely. I suspect this is because comment-reader was hard-coded to use `unsyntax' as the escape-id back into scribble.

This patch enables specifying a different escape-id in scribble/comment-reader. I'm not sure this is the best way to do it, but I like to submit patches with my feature requests.

I've updated the manual docs with an example that serves as a test. It will result in an error without this patch.