Update documentation on parallel readout errors (#4076)
Update documentation on parallel readout errors (#4076)

- Updating documentation on parallel readout errors
to clarify the methodology of sampling many different input
bitstrings and measuring appropriately.
- Documentation content provided by bfoxen@.
quantumlib/Cirq,quantumlib/Cirq,quantumlib/Cirq,quantumlib/Cirq,quantumlib/Cirq