Improve docstrings for functions and classes

[VeckoTheGecko]

The API reference for Parcels is a great way to understand how to use individual functions. Some of the functions/classes don't have documentation as the Python docstring is empty, meaning that users either have to look at documentation notebooks for examples, or look at the code itself. It would be good to update these docstrings with a description or (if more appropriate/easier) update the docstring with a link to the best tutorial to learn about said function.

If linking to a documentation notebook make sure to use the https://.../latest/.... link rather than tying to a version so that if the link is broken in future we can have that picked up by our automations.