change pval_adj docs

[jraffa]

Minor change to the documentation:

        The P-values from the unadjusted table (default when pval=True)
        are adjusted to account for the number of total tests that were performed.
        These adjustments would be useful when many variables are being screened
        to assess if their distribution varies by the variable in the groupby argument.

[tompollard]

Looks good, thanks!