Refactor bus documentation

PR #2962 changes the way interaction groups are packed into different columns, invalidating the existing description in the docs.

The new abstraction introduces language such as columns, groups, and batches, and relies on mutual exclusivity of selectors to achieve more optimal column packing. These are the important concepts that must be documented, rather than the interactions themselves, which are now self-documented in the code. More precisely, the framework requires each interaction to describe its name, along with the expected degree of the numerator and denominator of the fraction.