Document that the 'css' property no longer works after upgrading from layout plugin

Problem/Motivation

According to How to register layouts with Layout Plugin and How to upgrade from Layout Plugin, a layout can provide a CSS like this:

one_column:
  label: One column
  category: My Layouts
  template: templates/one-column
  css: css/one-column.css
  regions:
    main:
      label: Main content

However, the CSS key does not work anymore, leading e.g. to #2872954: Drupal 8.3 Layout system breaks layouts CSS.
Layout discovery itself also does explicitly declare libraries in core/modules/layout_discovery/layout_discovery.libraries.yml

Long story short: Existing yml defined layouts break.

As a major upgrade repercussion setting major.

Proposed resolution

Add the css key recognition like in old layout module.

OR: Explain why and update How to upgrade from Layout Plugin

Remaining tasks

Code, test, commit.

User interface changes

None.

API changes

Pure addition.

Data model changes

None.