Update docs for ApproxTimeEvolution to reference TrotterProduct (#…

…4679)

**Context:**
Once the new `TrotterProduct` functionality is added to Pennylane, the
`ApproxTimeEvolution` class will be redundant. In future releases it
will be deprecated and removed. For now we point to the new
functionality in the doc string.

pennylane/templates/subroutines/approx_time_evolution.py

@@ -24,6 +24,12 @@ class ApproxTimeEvolution(Operation):
     r"""Applies the Trotterized time-evolution operator for an arbitrary Hamiltonian, expressed in terms
     of Pauli gates.
 
+    .. note::
+
+        This operation is a special case of :class:`~pennylane.TrotterProduct` and will be deprecated in
+        future releases, please use :class:`~pennylane.TrotterProduct` as the preferred operation for this
+        functionality.
+
     The general time-evolution operator for a time-independent Hamiltonian is given by
 
     .. math:: U(t) \ = \ e^{-i H t},