doc: explain working with time-based index in functions
This is the idea which is waiting to be converted into a proper PR:
https://github.com/timmie/pandas/blob/9b0932368d6cf608f9e39d14a9532754f672d410/doc/source/timeseries.rst

If you like to have such a section as well, you can assign me and I will finally prepare the PR.
