Added an explanation on how to create a bit flip oracle from a phase …

…flip oracle

qiskit/circuit/classicalfunction/__init__.py

@@ -130,7 +130,8 @@ def grover_oracle(a: Int1, b: Int1, c: Int1, d: Int1) -> Int1:
     removal_timeline="in Qiskit 2.0",
     additional_msg="Use `PhaseOracle` instead, which can be turned into a "
     "bit-flip oracle by applying Hadamard gates on the target "
-    "qubit before and after the instruction.",
+    "qubit before and after the instruction, and conditioning."
+    "the instruction on the target qubit.",
 )
 def classical_function(func):
     """

qiskit/circuit/classicalfunction/boolean_expression.py

@@ -30,7 +30,8 @@ class BooleanExpression(ClassicalElement):
         removal_timeline="in Qiskit 2.0",
         additional_msg="Use `PhaseOracle` instead, which can be turned into a "
         "bit-flip oracle by applying Hadamard gates on the target "
-        "qubit before and after the instruction.",
+        "qubit before and after the instruction, and conditioning."
+        "the instruction on the target qubit.",
     )
     def __init__(self, expression: str, name: str = None, var_order: list = None) -> None:
         """

qiskit/circuit/classicalfunction/classicalfunction.py

@@ -33,7 +33,8 @@ class ClassicalFunction(ClassicalElement):
         removal_timeline="in Qiskit 2.0",
         additional_msg="Use `PhaseOracle` instead, which can be turned into a "
         "bit-flip oracle by applying Hadamard gates on the target "
-        "qubit before and after the instruction.",
+        "qubit before and after the instruction, and conditioning."
+        "the instruction on the target qubit.",
     )
     def __init__(self, source, name=None):
         """Creates a ``ClassicalFunction`` from Python source code in ``source``.

releasenotes/notes/deprecate-classical-function-3bf44ef26d984366.yaml

@@ -10,3 +10,16 @@ deprecations_circuits:
     For a similar functionality please use the :class:`.PhaseOracle` which
     is going to have a `tweedledum` independent implementation, and
     the :class:`.BitFlipOracle` which will be added in Qiskit 2.0.
+
+    Until :class:`.BitFlipOracle` is added, a phase flip oracle can be converted
+    to a bit flip oracle by conditioning it on the result qubit, and applying 
+    Hadamard gate before and after the application of the oracle, as in the
+    following example (where the oracle is on `qr_x` and the result is on `qr_y`):
+
+    .. code-block:: python
+    
+        phase_flip_oracle = PhaseOracle(bool_expr)
+        controlled_phase_flip_oracle = phase_flip_oracle.control(1)
+        qc.h(qr_y)
+        qc.compose(controlled_phase_flip_oracle, qubits=[*qr_y, *qr_x], inplace=True)
+        qc.h(qr_y)