Add some high-level docs to FnCtxt and ItemCtxt

I haven't understood the difference between these before, but
`@compiler-errors` helped me clear it up. Hopefully this will help other
people who've been confused!

Author: camelid

compiler/rustc_typeck/src/check/fn_ctxt/mod.rs

@@ -26,6 +26,8 @@ use rustc_trait_selection::traits::{ObligationCause, ObligationCauseCode};
 use std::cell::{Cell, RefCell};
 use std::ops::Deref;
 
+/// The `FnCtxt` stores type-checking context needed to type-check function bodies,
+/// in contrast to [`ItemCtxt`], which is used to type-check item *signatures*.
 pub struct FnCtxt<'a, 'tcx> {
     pub(super) body_id: hir::HirId,
 

compiler/rustc_typeck/src/collect.rs

@@ -94,7 +94,12 @@ pub fn provide(providers: &mut Providers) {
 ///////////////////////////////////////////////////////////////////////////
 
 /// Context specific to some particular item. This is what implements
-/// `AstConv`. It has information about the predicates that are defined
+/// `AstConv`.
+///
+/// `ItemCtxt` is primarily used to type-check item signatures, in contrast to [`FnCtxt`],
+/// which is used to type-check function bodies.
+///
+/// It has information about the predicates that are defined
 /// on the trait. Unfortunately, this predicate information is
 /// available in various different forms at various points in the
 /// process. So we can't just store a pointer to e.g., the AST or the