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.

src/libsyntax/ext/deriving/mod.rs

@@ -9,10 +9,10 @@
 // except according to those terms.
 
 /*!
-The compiler code necessary to implement the #[deriving] extensions.
+The compiler code necessary to implement the `#[deriving]` extensions.
 
 
-FIXME (#2810)--Hygiene. Search for "__" strings (in other files too).
+FIXME (#2810): hygiene. Search for "__" strings (in other files too).
 We also assume "extra" is the standard library, and "std" is the core
 library.