Clarify the CONTRIBUTING.md section on stream capture (#5761)
Clarify the CONTRIBUTING.md section on stream capture (#5761)

This commit expands on the stream capture section of the CONTRIBUTING.md
guide. The existing section was a bit confusing because it makes the
QISKIT_TEST_CAPTURE_STREAMS environment variable sound general but it
only works with stestr (or a similar test runner built on the same
stack). This clarifies that it only has the desired effect with stestr
and if you're using the stdlib runner this is the equivalent of running
--buffer.
QISKit/qiskit-sdk-py,QISKit/qiskit-sdk-py,QISKit/qiskit-sdk-py