Removes pallet::call_index from dev_mode example (#14492)

* Removes call_index

* Adds UI test and updates doc

* Update frame/examples/dev-mode/src/lib.rs

Co-authored-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>

* Update frame/examples/dev-mode/src/lib.rs

Co-authored-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>

---------

Co-authored-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>

substrate/frame/support/test/tests/pallet_ui/dev_mode_without_arg_call_index.rs

@@ -0,0 +1,31 @@
+#![cfg_attr(not(feature = "std"), no_std)]
+
+pub use pallet::*;
+
+#[frame_support::pallet]
+pub mod pallet {
+	use frame_support::pallet_prelude::*;
+	use frame_system::pallet_prelude::OriginFor;
+
+	// The struct on which we build all of our Pallet logic.
+	#[pallet::pallet]
+	pub struct Pallet<T>(_);
+
+	// Your Pallet's configuration trait, representing custom external types and interfaces.
+	#[pallet::config]
+	pub trait Config: frame_system::Config {}
+
+	// Your Pallet's callable functions.
+	#[pallet::call]
+	impl<T: Config> Pallet<T> {
+		#[pallet::weight(0)]
+		pub fn my_call(_origin: OriginFor<T>) -> DispatchResult {
+			Ok(())
+		}
+	}
+
+	// Your Pallet's internal functions.
+	impl<T: Config> Pallet<T> {}
+}
+
+fn main() {}

substrate/frame/support/test/tests/pallet_ui/dev_mode_without_arg_call_index.stderr

@@ -0,0 +1,24 @@
+error: use of deprecated constant `pallet::warnings::ImplicitCallIndex_0::_w`:
+               It is deprecated to use implicit call indices.
+               Please instead ensure that all calls have a `pallet::call_index` attribute or put the pallet into `dev` mode.
+
+               For more info see:
+                   <https://github.com/paritytech/substrate/pull/12891>
+                   <https://github.com/paritytech/substrate/pull/11381>
+  --> tests/pallet_ui/dev_mode_without_arg_call_index.rs:22:10
+   |
+22 |         pub fn my_call(_origin: OriginFor<T>) -> DispatchResult {
+   |                ^^^^^^^
+   |
+   = note: `-D deprecated` implied by `-D warnings`
+
+error: use of deprecated constant `pallet::warnings::ConstantWeight_0::_w`:
+               It is deprecated to use hard-coded constant as call weight.
+               Please instead benchmark all calls or put the pallet into `dev` mode.
+
+               For more info see:
+                   <https://github.com/paritytech/substrate/pull/13798>
+  --> tests/pallet_ui/dev_mode_without_arg_call_index.rs:21:20
+   |
+21 |         #[pallet::weight(0)]
+   |                          ^