I think it would be better if the requirements on x_min and x_max are described more explicitly:  
"The function can be a callable function, in which case specifying the parameters x_min and x_max values is required, or the name of a file containing two columns, x and P(x), in which case specifying x_min and x_max would result in an error."

Perhaps mention the use of `npoints` as well, since it's for generating the cumulative distribution function, etc, so that it shouldn't be set to the number of points contained in the `x, P(x)` table, which was my initial guess....  A more descriptive variable name might help, but I can't think of an good one.