Rollup merge of rust-lang#134787 - fmease:spruce-up-queries, r=compiler-errors

Spruce up the docs of several queries related to the type/trait system and const eval

- Editorial
  - Proper rustdoc summary/synopsis line by making use of extra paragraphs: Leads to better rendered output on module pages, in search result lists and overall, too
  - Use rustdoc warning blocks for admonitions of the form "do not call / avoid calling this query directly"
  - Use intra-doc links of the form ``[`Self::$query`]`` to cross-link queries. Indeed, such links are generally a bit brittle due to the existence of `TyCtxtFeed` which only contains a subset of queries. Therefore the docs of `feedable` queries cannot cross-link to non-`feedable` ones. I'd say it's fine to use intra-doc links despite the potential/unlikely occasional future breakage (if a query with the aforementioned characteristics becomes `feedable`). `Self::` is nicer than `TyCtxt::` (which would be more stable) since it accounts for other contexts like `TyCtxt{Feed,At,Ensure{,WithValue}}`
- Informative
  - Generally add, flesh out and correct some doc comments
  - Add *Panic* sections (to a few selected queries only). The lists of panics aren't necessarily exhaustive and focus on the more "obvious" or "important" panics.
  - Where applicable add a paragraph calling attention to the relevant [`#[rustc_*]` TEST attribute](https://rustc-dev-guide.rust-lang.org/compiler-debugging.html#rustc_-test-attributes)

The one non-doc change (it's internal and not observable):
Be even more defensive in `query constness`'s impl (spiritual follow-up to rust-lang#134122) (see self review comment).

Fixes rust-lang#133494.

r\? **any**(compiler-errors, oli-obk)

Author: jieyouxu

compiler/rustc_const_eval/src/const_eval/fn_queries.rs

@@ -15,20 +15,14 @@ fn parent_impl_constness(tcx: TyCtxt<'_>, def_id: LocalDefId) -> hir::Constness
     }
 }
 
-/// Checks whether an item is considered to be `const`. If it is a constructor, it is const.
-/// If it is an assoc method or function,
-/// return if it has a `const` modifier. If it is an intrinsic, report whether said intrinsic
-/// has a `rustc_const_{un,}stable` attribute. Otherwise, panic.
+/// Checks whether a function-like definition is considered to be `const`.
 fn constness(tcx: TyCtxt<'_>, def_id: LocalDefId) -> hir::Constness {
     let node = tcx.hir_node_by_def_id(def_id);
 
     match node {
-        hir::Node::Ctor(hir::VariantData::Tuple(..))
-        | hir::Node::ImplItem(hir::ImplItem { kind: hir::ImplItemKind::Const(..), .. }) => {
-            hir::Constness::Const
-        }
-        hir::Node::ForeignItem(_) => {
-            // Foreign items cannot be evaluated at compile-time.
+        hir::Node::Ctor(hir::VariantData::Tuple(..)) => hir::Constness::Const,
+        hir::Node::ForeignItem(item) if let hir::ForeignItemKind::Fn(..) = item.kind => {
+            // Foreign functions cannot be evaluated at compile-time.
             hir::Constness::NotConst
         }
         hir::Node::Expr(e) if let hir::ExprKind::Closure(c) = e.kind => c.constness,