EntityDisplayInterface::setComponent() documentation is incomplete

Problem/Motivation

Several problems:

1. The $options is not documented as optional
2. EntityDisplayBase does various things which are not documented:

- // If no weight specified, make sure the field sinks at the bottom.
- // For a field, fill in default options.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet