Although commit 5415cca336 just made run-time checking consistent with
the documentation, the implementation turned out to allow content
before, and some existing documents relied on that. Changes the
documentation and contract to allow content, instead.
Scribble used to support a custom style for optional brackets.
In particular, the optional brackets will be given the `'paren-shape`
of value `#\?`[1], and the CSS class `opt-color` will be tagged
to these brackets.
Later, Scribble supports the curry notation. Its implementation
no longer uses `'paren-shape` to indicate optional brackets.
Unfortunately, it accidentally dropped the `opt-color` tagging[2].
This PR restores the original behavior by tagging the class
`opt-color` to optional brackets. It also adds `racketoptionalfont`
so that Scribble users can typeset optional brackets.
Lastly, it cleans up the code that supports the `'paren-shape` of value
`#\?`, since it is effectively a deadcode.
Note that this PR does _not_ change any CSS styling, so there's no
visible change. It would make CSS styling customization easier, however.
[1]: 9b7993ea02 (diff-017add06555fc85fa3ae5f27a3eb52cbR253)
[2]: https://github.com/racket/scribble/commit/95ecb101d1cc61d212c4d520#diff-017add06555fc85fa3ae5f27a3eb52cbR879
The indentation for multi-line typesetting is weird, because it still
uses the table-basd layout that lines up all arguments independent of
nesting. But at least the parentheses are not broken.
Relevant to #211
