[meta] Document @internal APIs both explicitly in phpdoc and implicitly in d.o documentation

Problem/Motivation

Drupal 8 core has an established backwards compatibility and internal API policy → and a deprecation process → , but the @internal tags have not been documented explicitly everywhere, and some kinds of pseudo-API are not yet covered.

Proposed resolution

Open sub-issues for things not covered. Add children to #2873395: [meta] Add @internal to core classes, methods, properties → for things documented on the policy but not in the codebase.

Remaining tasks

User interface changes

API changes

Data model changes