Add note to is_known_rigid

compiler/rustc_middle/src/ty/sty.rs

@@ -2946,6 +2946,11 @@ impl<'tcx> Ty<'tcx> {
         }
     }
 
+    /// Whether the outermost type cannot be further normalized, resolved, or
+    /// substituted. This includes all primitive types, but also things like
+    /// ADTs and trait objects, sice even if their arguments or nested types
+    /// may be further simplified, the outermost [`TyKind`] or type constructor
+    /// remains the same.
     pub fn is_known_rigid(self) -> bool {
         match self.kind() {
             Bool