Fix syntax::ext::deriving{,::*} docs formatting.

Fix syntax::ext::deriving{,::*} docs formatting.

The most significant fix is for `syntax::ext::deriving::encodable`,
where one of the blocks of code, auspiciously containing `<S>` (recall
that Markdown allows arbitrary HTML to be contained inside it), was not
formatted as a code block, with a fun but messy effect.