Document pi_check correctly

The docstring for pi_check is inaccurate.

To fix this, one needs to read and test the code to see what the code does. Finding what the code does and expressing it in a doc string is relatively easy. But we need to document the part we want to guarantee. Possibly make some changes if it does not do what we really want.

See also

• #10197
• #9238
• #8778
• #4447

Version

qiskit-terra cd770b59