JSON:API's notice about no PHP API is no longer valid.

Problem/Motivation

The current jsonapi classes contain this string:

" * @internal JSON:API maintains no PHP API. The API is the HTTP API. This class
* may change at any time and could break any dependencies on it."

But this is not really the case anymore. There are a few classes that are used to define a public API through events.

This is also a problem in jsonapi.api.php which is no longer the truth.

* The JSON:API module provides *no PHP API to modify its behavior.* It is
 * designed to have zero configuration.
 *
 * - Adding new resources/resource types is unsupported: all entities/entity
 *   types are exposed automatically. If you want to expose more data via the
 *   JSON:API module, the data must be defined as entity. See the "Resources"
 *   section.
 * - Custom field type normalization is not supported because the JSON:API
 *   specification requires specific representations for resources (entities),
 *   attributes on resources (non-entity reference fields) and relationships
 *   between those resources (entity reference fields). A field contains
 *   properties, and properties are of a certain data type. All non-internal
 *   properties on a field are normalized.
 * - The same data type normalizers as those used by core's Serialization and
 *   REST modules are also used by the JSON:API module.
 * - All available authentication mechanisms are allowed.

Steps to reproduce

n.a.

Proposed resolution

We need new wording which makes clear that there is an API, but everything @internal is REALLY internal as mentioned above.

Remaining tasks

1. Decide on a message
2. Decide on what is missing and wrong in jsonapi.api.php
3. Implement the messages

User interface changes

None

API changes

None

Data model changes

None

Release notes snippet