Rollup merge of rust-lang#116041 - compiler-errors:rigid-note, r=RalfJung

Add note to `is_known_rigid`

Adds a note requested by `@RalfJung` in rust-lang#114941 (comment)

Let me know if there are any other fns that need documentation, I could throw them into this PR too :)

Author: matthiaskrgr

Diff for: compiler/rustc_middle/src/ty/sty.rs

@@ -2946,6 +2946,11 @@ impl<'tcx> Ty<'tcx> {
         }
     }
 
+    /// Returns `true` when 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