Update (doc) comments

Several (doc) comments were super outdated or didn't provide enough context.

Some doc comments shoved everything in a single paragraph without respecting
the fact that the first paragraph should be a single sentence because rustdoc
treats these as item descriptions / synopses on module pages.

compiler/rustc_ast/src/ast.rs

@@ -303,10 +303,6 @@ impl TraitBoundModifiers {
     };
 }
 
-/// The AST represents all type param bounds as types.
-/// `typeck::collect::compute_bounds` matches these against
-/// the "special" built-in traits (see `middle::lang_items`) and
-/// detects `Copy`, `Send` and `Sync`.
 #[derive(Clone, Encodable, Decodable, Debug)]
 pub enum GenericBound {
     Trait(PolyTraitRef, TraitBoundModifiers),