with_probability docstring (quantumlib#5697)

Adds a docstring for `Operation.with_probability`.

cirq-core/cirq/ops/raw_types.py

@@ -345,8 +345,8 @@ def with_probability(self, probability: 'cirq.TParamVal') -> 'cirq.Gate':
         """Creates a probabalistic channel with this gate.
 
         Args:
-            probability: floating value between 0 and 1, giving the probability
-                this gate is applied.
+            probability: floating point value between 0 and 1, giving the
+                probability this gate is applied.
 
         Returns:
             `cirq.RandomGateChannel` that applies `self` with probability
@@ -579,6 +579,19 @@ def controlled_by(
         return ops.controlled_operation.ControlledOperation(control_qubits, self, control_values)
 
     def with_probability(self, probability: 'cirq.TParamVal') -> 'cirq.Operation':
+        """Creates a probabalistic channel with this operation.
+
+        Args:
+            probability: floating point value between 0 and 1, giving the
+                probability this gate is applied.
+
+        Returns:
+            `cirq.RandomGateChannel` that applies `self` with probability
+                `probability` and the identity with probability `1-p`.
+
+        Raises:
+            NotImplementedError: if called on an operation that lacks a gate.
+        """
         gate = self.gate
         if gate is None:
             raise NotImplementedError("with_probability on gateless operation.")