doc: update obsolete docstring for registerDerivingHandler (#9152)

This PR fixes an obsolete docstring for `registerDerivingHandler`

src/Lean/Elab/Deriving/Basic.lean

@@ -67,11 +67,12 @@ def DerivingHandler := (typeNames : Array Name) → CommandElabM Bool
 
 builtin_initialize derivingHandlersRef : IO.Ref (NameMap (List DerivingHandler)) ← IO.mkRef {}
 
-/-- A `DerivingHandler` is called on the fully qualified names of all types it is running for
-as well as the syntax of a `with` argument, if present.
+/--
+Registers a deriving handler for a class. This function should be called in an `initialize` block.
 
-For example, `deriving instance Foo with fooArgs for Bar, Baz` invokes
-``fooHandler #[`Bar, `Baz] `(fooArgs)``. -/
+A `DerivingHandler` is called on the fully qualified names of all types it is running for. For
+example, `deriving instance Foo for Bar, Baz` invokes ``fooHandler #[`Bar, `Baz]``.
+-/
 def registerDerivingHandler (className : Name) (handler : DerivingHandler) : IO Unit := do
   unless (← initializing) do
     throw (IO.userError "failed to register deriving handler, it can only be registered during initialization")