Clarify wieght traits needed to impl in example

[nuke-web3]

Discovered inconsistency with devhub here: polkadot-developers/substrate-developer-hub.github.io#1125

Add a required third trait in the docs as evidenced here:

substrate/frame/example/src/lib.rs

Lines 324 to 349 in eded990

	struct WeightForSetDummy<T: pallet_balances::Config>(BalanceOf<T>);
	impl<T: pallet_balances::Config> WeighData<(&BalanceOf<T>,)> for WeightForSetDummy<T> {
	fn weigh_data(&self, target: (&BalanceOf<T>,)) -> Weight {
	let multiplier = self.0;
	// *target.0 is the amount passed into the extrinsic
	let cents = *target.0 / <BalanceOf<T>>::from(MILLICENTS);
	(cents * multiplier).saturated_into::<Weight>()
	}
	}
	impl<T: pallet_balances::Config> ClassifyDispatch<(&BalanceOf<T>,)> for WeightForSetDummy<T> {
	fn classify_dispatch(&self, target: (&BalanceOf<T>,)) -> DispatchClass {
	if *target.0 > <BalanceOf<T>>::from(1000u32) {
	DispatchClass::Operational
	} else {
	DispatchClass::Normal
	}
	}
	}
	impl<T: pallet_balances::Config> PaysFee<(&BalanceOf<T>,)> for WeightForSetDummy<T> {
	fn pays_fee(&self, _target: (&BalanceOf<T>,)) -> Pays {
	Pays::Yes
	}
	}

[gui1117]

we should probably also improve the doc in frame/support/src/lib.rs when explaining about the pallet::weight attribute.

[gui1117]

bot merge

[paritytech-processbot]

Waiting for commit status.