Add note to is_known_rigid

Author: compiler-errors

compiler/rustc_middle/src/ty/sty.rs

@@ -2946,6 +2946,10 @@ impl<'tcx> Ty<'tcx> {
         }
     }
 
+    /// Whether the outermost type cannot be further normalized, resolved, or
+    /// substituted. This includes all built-in types, but also things like
+    /// ADTs and dynamic types, even if their arguments or nested types may be
+    /// further simplified.
     pub fn is_known_rigid(self) -> bool {
         match self.kind() {
             Bool