Sundown does not support the definition lists (and affects Rust documents)

[lifthrasiir]

Sundown, and its proposed successor, Hoedown (cf. #12776), does not support the definition lists unlike Pandoc. src/doc/rust.md still contains lots of definition lists (search for /^ *: /) and they get rendered completely unreadable. (To my knowledge, it is the only document with definition lists, but I may be wrong.)