Document CircuitOp optimizer best practices (#5221)

Final item for #3634.

With the new transformers, there is a clean pipeline for optimizing nested circuits and sending the (concise!) result to hardware for execution.

Author: 95-martin-orion

docs/google/best_practices.md

@@ -56,9 +56,6 @@ sent to Quantum Engine due to an upper limit on request size. If the circuits
 in question have a repetitive structure, `cirq.CircuitOperation`s can be used
 to reduce the request size and avoid this limit.
 
-`optimized_for_sycamore` will preserve `CircuitOperation`s while optimizing
-their contents.
-
 ```python
 import cirq
 import cirq_google as cg
@@ -81,6 +78,20 @@ circuit_op = circuit_op.repeat(100)
 short_circuit = cirq.Circuit(circuit_op for q in qubits)
 ```
 
+When compiling circuits with `CircuitOperation`s, providing a context
+with `deep=True` will preserve the `CircuitOperation`s while
+optimizing their contents. This is useful for producing a concise,
+device-compatible circuit.
+
+```python
+ctx = cirq.TransformerContext(deep=True)
+cz_circuit = cirq.optimized_for_target_gateset(
+    short_circuit,
+    cirq.CZTargetGateset,
+    context=ctx
+)
+```
+
 
 ## Running circuits faster