Improve error message on where clause on pallet error (#7821)

* improve error message on where clause on pallet error

* Revert "improve error message on where clause on pallet error"

This reverts commit 5a3cc38976813fccef3357833553ce30f5b988ea.

* Revert "Revert "improve error message on where clause on pallet error""

This reverts commit e3b3fca6bc4fa89816f80dbcb82dc4536a9b2549.

substrate/frame/support/src/lib.rs

@@ -1256,6 +1256,8 @@ pub mod pallet_prelude {
 /// }
 /// ```
 /// I.e. a regular rust enum named `Error`, with generic `T` and fieldless variants.
+/// The generic `T` mustn't bound anything and where clause is not allowed. But bounds and where
+/// clause shouldn't be needed for any usecase.
 ///
 /// ### Macro expansion
 ///