grammar should not be looking for # characters in freeform comments

[pickx]

the fact that a freeform comment should contain # that might later be passed to a Markdown parser, is an implementation detail.

this means:

1. we should not treat header and body separately, and we should not special case the Markdown-related characters. it's just text.
2. there is no reason to special case single-line and multi-line freeform comments. again, these are just blocks of text, possibly separated by newline characters.
3. we should not attempt to clean # characters from the comment text, that's work that the user should be doing.

a consequence of this is whether or not we're supposed to clean leading # characters in headers. the documentation seems to indicate that we should. for example /**** # Section example *************************/ should be parsed to "Multiline example", trimming both the leading whitespace and the #, but this no longer seems like the right call.