Differential entropy docstrings: simplify the Shannon-based ones?

[kahaaga]

As discussed in #230, we could shorten the docstrings for the Shannon differential entropy estimators by just stating the formula once in the overall docs.

But there will also be differential entropy estimators that estimates Tsallis and Renyi entropy, which will display other formulas. Should we display formulas for none or all docstring? Or just some?

There are arguments for and against doing so. We should re-assess this once the number of estimators has grown a bit, and we cover more EntropyDefinitions