Need documentation on what protobuf serializations are allowed

[peterse]

It is important for users to know if their sympy-parametrized circuits will be protobuf serializable, either through more verbose error messages or more thorough documentation on which sympy features are supported.

xref #2122

[verult]

Once #5050 lands, a circuit can then be validated against a device before serialization by using the device's gateset: engine.get_processor('processor').get_device().metadata.gateset.validate(circuit).

Printing the gateset will also show the full supported gateset of the device.

[maffoo]

@verult, note that the question was about what sympy features are supported by serialization. We only support a very limited set of sympy expressions such as addition, multiplication, and exponentiation, not arbitrary sympy expressions. This probably doesn't need to be part of gateset, though, but rather should be part of the other validate we do before sending to QCS.

[verult]

Gotcha. I will update the documentation on this limitation (maybe here) and look into adding validation.