Update the API documentation of MultiIndexSet

A (first?) big round of refactoring the MultiIndexSet class is at an end (see Issue #99 (closed)). To finally close this issue, the documentation of MultiIndexSet class need to be updated.

Currently, the API reference for the class starts with:

construction: Here we need a description of multi index sets 🚧

The class is established enough to have its own more complete description. By updating the module-level docstrings, the documentation will be injected into the HTML document.

Furthermore, other in-code documentation within the same module (mainly, the class itself) should be reviewed and updated if necessary.