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Several (doc) comments were super outdated or didn't provide enough context.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_span/src/hygiene.rs

@@ -1062,7 +1062,7 @@ pub enum ExpnKind {
     Macro(MacroKind, Symbol),
     /// Transform done by the compiler on the AST.
     AstPass(AstPass),
-    /// Desugaring done by the compiler during HIR lowering.
+    /// Desugaring done by the compiler during AST lowering.
     Desugaring(DesugaringKind),
 }