Scribble Latex/PDF: fix quote and backquote rendering

When text is literally a quote (ASCII 39) or backquote (ASCII 96),
then render it in a way that prevents LaTeX from converting to
curly variants.
scribble: fix documentation for `tt` and `racketfont`

The `tt` and `racketfont` functions treat their arguments strangely,
applying styles only to immediate strings and symbols.

Add `racketplainfont`, which is a non-strange version of `racketfont`.