Accept any input when parsing documentation

[SimonHengel]

The parser for documentation should never fail. If something is no valid Haddock syntax, then it can still be parsed as ordinary text.

E.g. the following is currently a parse error:

foo bar
> baz

But we should instead parse it as:

DocParagraph (DocString "foo bar\n> baz\n")

That's the way most Markdown parsers do it.