More detailed method descriptions for package

[dmtrung14]

As a newbie, I have some problems understanding a few of the descriptions. Some of the methods could be documented based on their actions and the expected results.

• the MixedDestabilizer could be phrased as:
  "Keeping track of the r constraints over n qubits, resulting in r qubits that trigger check on the unique corresponding constraint in the matrix, and (n-r) qubits that do not trigger check in any of the constraints"

• the sMZ gate, make_noisy, and UnbiasedUncorrelatedNoise can also be defined more specifically