Fix docs typos (#463)

Author: gdalle

DifferentiationInterface/src/second_order/hessian.jl

@@ -11,7 +11,7 @@ Create an `extras` object that can be given to [`hessian`](@ref) and its variant
 function prepare_hessian end
 
 """
-    hessian(f, backend, x, [extras]) -> hess
+    hessian(f, [extras,] backend, x) -> hess
 
 Compute the Hessian matrix of the function `f` at point `x`.
 
@@ -20,7 +20,7 @@ $(document_preparation("hessian"))
 function hessian end
 
 """
-    hessian!(f, hess, backend, x, [extras]) -> hess
+    hessian!(f, hess, [extras,] backend, x) -> hess
 
 Compute the Hessian matrix of the function `f` at point `x`, overwriting `hess`.
 

DifferentiationInterface/src/second_order/hvp.jl

@@ -30,7 +30,7 @@ $(document_preparation("hvp"; same_point=true))
 function hvp end
 
 """
-    hvp!(f, dg, [extras,] backend, x, tx) -> tg
+    hvp!(f, tg, [extras,] backend, x, tx) -> tg
 
 Compute the Hessian-vector product of `f` at point `x` with tangent `tx` of type [`Tangents`](@ref), overwriting `tg`.