Improve separate_circuit docstring

[garrison]

I've also added circuit_knitting/ to the paths passed to pytest, as this is required for doctests to be discovered.

[garrison]

Without the _ = , one gets extra output from the non-None expressions:

>>> from qiskit import QuantumCircuit
>>> qc = QuantumCircuit(4)
>>> qc.x(0)
<qiskit.circuit.instructionset.InstructionSet object at 0x7f710e9921c0>
>>> qc.cx(1, 2)
<qiskit.circuit.instructionset.InstructionSet object at 0x7f70da3873a0>
>>> qc.h(3)
<qiskit.circuit.instructionset.InstructionSet object at 0x7f70da3872b0>

and doctest fails unless these are also added here. Of course, the memory address is not predictable, so there is no way to allow such a thing to be output and still have the tests pass.

[github-actions]

Pull Request Test Coverage Report for Build 5295854068

• 3 of 3 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 89.638%

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Totals
Change from base Build 5295429418:	0.0%
Covered Lines:	2327
Relevant Lines:	2596

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💛 - Coveralls

[garrison]

This file had to be modified for doctest discovery to work on Python 3.11 (i.e., when DOcplex is not present).

[caleb-johnson]

Awesome, much needed, thanks. Thanks for adding it to api docs as well. I approved with one minor language suggestion