Where to find statistically relevant documentation of common Python packages?
I think the documentation, at least for your example, is not too obscure. It doesn't tell you what kind of interpolation it uses because it doesn't use any: resample is a deferred operation. In order for it to work you have to call it in conjunction with a function that performs the interpolation, e.g. series.resample(frequency).mean() or series.resample(frequency).interpolation('cubic') (as can be seen from the examples in the documentation). Sometimes, it can be helpful to also look into the release notes for additional information.

Scipy and Numpy are sometimes not very detailed, but they contain at least a decent number of references with more information. (There is also a section on Matlab versus Numpy here. If you are familiar with dplyr in R , then the pandas cheat sheet can help as a quick comparison between R and pandas for typical data wrangling operations.) 

I'm not aware of a "second more detailed documentation" that encompasses the entire libraries. You probably have to fall back to books (which, of course, aren't complete or always up to date).