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Several (doc) comments were super outdated or didn't provide enough context.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_hir_analysis/src/variance/constraints.rs

@@ -1,6 +1,6 @@
 //! Constraint construction and representation
 //!
-//! The second pass over the AST determines the set of constraints.
+//! The second pass over the HIR determines the set of constraints.
 //! We walk the set of items and, for each member, generate new constraints.
 
 use hir::def_id::{DefId, LocalDefId};