docs for return values from various EntityDisplayRepositoryInterface() are unclear

Problem/Motivation

Several methods on EntityDisplayRepositoryInterface - all the get*Modes() - have @return docs that say something like this:

   * @return array
   *   The view mode info for all entity types.

What is the 'info'? What is the array keyed by? The docs should say this.

Steps to reproduce

Proposed resolution

Complete the documentation.

It's probably fine for the entity type-specific flavours to refer to the general method for docs, to avoid repetition.

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet