Add @internal to core classes, methods, properties to Underscore-prefixed functions and methods

Follow-up to #2873395: [meta] Add @internal to core classes, methods, properties →

Problem/Motivation

https://www.drupal.org/core/d8-bc-policy#underscore →
Functions or methods with an underscore prefix (e.g. _some_function()) are considered internal, and may be used to avoid name collisions when backporting protected methods critical or major bug fixes. Extending code should not call these functions or methods directly nor use the underscore prefix for added methods.

Proposed resolution

Add @internal tag to the the underscore prefixed functions and methods.

How to fix this issue

1. Read the description for a category in its issue
2. Identify and confirm an example. Ask in IRC if unclear.
3. Search core for the relevant category.
4. Add @internal per the backwards compatibility policy.
5. Reviewers should confirm that each @internal mention is appropriate for that category according to the policy.

Remaining tasks

Make the Patch and post it

User interface changes

None.

API changes

There should be no implicit API changes.