Consistent formatting for `InformationMeasure`s

[kahaaga]

Some of the entropy and extropy definitions have really nice docstrings that clearly outline all formulas, assumptions and meanings of symbols. The Renyi and Tsallis entropies are good examples.

However, several of the information measures don't explain what symbols mean and are formatted differently (without a ## Description header). It would be nice to have consistent formatting and explanations.