Clarify in docstrings what an expected CLs value is

[matthewfeickert]

Hi, this clarifies why using CLs_exp from pyhf does not give the result we expect, but it contradicts what we understand as expected values (used to evaluate, e.g., the sensitivity of an analysis). IMHO spelling this out in the documentation would be useful, as we (in this case the MadAnalysis5 team) are likely not the only ones who meet this pitfall.

Originally posted by @sabinekraml in #1619 (reply in thread)