fix most compiler/ doctests

compiler/rustc_trait_selection/src/traits/select/mod.rs

@@ -1933,7 +1933,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
     ///
     /// Here are some (simple) examples:
     ///
-    /// ```
+    /// ```ignore (illustrative)
     /// (i32, u32) -> [i32, u32]
     /// Foo where struct Foo { x: i32, y: u32 } -> [i32, u32]
     /// Bar<i32> where struct Bar<T> { x: T, y: u32 } -> [i32, u32]