ArithmeticOperation examples render with a bunch of xml cruft

[maffoo]

Description of the issue

The docstring for ArithmeticOperation has an Examples: section with code in triple backticks: https://github.com/quantumlib/Cirq/blob/master/cirq-core/cirq/ops/arithmetic_operation.py#L43. This renders on the docsite with a bunch of XML cruft: https://quantumai.google/reference/python/cirq/ops/ArithmeticOperation#examples

[maffoo]

xref #5151