QSE missing documentation on finding/creating "check operators"

[natestemen]

Quantum Subspace Expansion (QSE) requires the user to specify a set of "check operators" ${M_i}$ which should leave the final state of the ideal circuit invariant $M_i\ket{\psi} = \ket{\psi}$.

There is a brief discussion about using the expansion of the generators of a specific QEC code in the How do I use QSE? section of the user guide, but there isn't any information if you aren't drawing inspiration from QEC codes.

Hence, depending on when QSE can be used there are two potential outcomes here.

1. QSE can be used with check operators that are not derived from a QEC code in which case we should add some guidance on how the check operators might be discovered.
2. QSE cannot be used with check operators outside the realm of QEC, in which case we should document this as a warning.