Add a docstring to testing default gate domain constant (#3047)

This was breaking my doc build for reasons I have not been able to track down.

Author: dabacon

cirq/testing/random_circuit.py

@@ -16,6 +16,7 @@
 
 from cirq import ops, value
 from cirq.circuits import Circuit
+from cirq._doc import document
 
 if TYPE_CHECKING:
     import cirq
@@ -33,6 +34,13 @@
     ops.Y: 1,
     ops.Z: 1
 }
+document(
+    DEFAULT_GATE_DOMAIN,
+    """The default gate domain for `cirq.testing.random_circuit`.
+
+This includes the gates CNOT, CZ, H, ISWAP, CZ, S, SWAP, T, X, Y,
+and Z gates.
+""")
 
 
 def random_circuit(qubits: Union[Sequence[ops.Qid], int],