Entity API documentation needs to explain what an "entity handler" is

Problem/Motivation

"Entity handlers" are classes that provide different sorts of functionality for entity types: storage, listing, access control, and so on. They used to be called "controllers" but were renamed because this caused confusion with routing controllers. See #1976158: Rename entity storage/list/form/render "controllers" to handlers.

Currently, however, there is not really any place in our documentation that explains what "handlers" are, and there are still some incorrect references to "controllers".

Proposed resolution

1. Update the Entity API documentation topic to refer to "handlers" instead of controllers where appropriate, and add a brief explanation of what a "handler" is.
2. Update the docblocks for EntityHandlerInterface and EntityHandlerBase to briefly explain what a "handler" is, and reference the group documentation for more detail. [Note: see comment #1, where it was suggested just to reference the group documentation for more detail.]
3. Check for other places in documentation that might still inappropriately refer to "controllers" that are actually entity handlers. Note that most references to "controller" in the Drupal codebase are correct and refer instead to routing controllers.

Remaining tasks

User interface changes

API changes

Beta phase evaluation