DOC: is confusing for ddof parameter of sem, var and std functions
https://github.com/pandas-dev/pandas/blob/7bb204a05fee20c3c825e7da39ccaf39fbeb8ca5/pandas/core/generic.py#L7160-L7161

It states that _ddof_ is for degrees of freedom with default value of 1. But actually it is for "Delta degrees-of-freedom" (even the name of the parameter is abbreviation of that definition). The meaning is:  how many degrees of freedom **to adjust** relative to the population size. Default of 1 - means we need to **adjust** by one, not use one as a value of degrees of freedom (thats makes no sense). All functions are worked as expected, but documentation on _ddof_ parameter is confusing.