Properly document the `$page_hook` action

[johnbillion]

The $page_hook action lacks meaningful documentation. This is the only action in core that uses a completely dynamic hook name, which means it needs to be well documented to help developers who may wish to use it.

Most of the documentation from the load-{$page_hook} action immediately above it can be reused, and then the @ignore tag can be removed so the unnecessary private flag gets removed from the developer reference.

Refs:

• ​https://developer.wordpress.org/reference/hooks/page_hook/
• ​https://github.com/WordPress/wordpress-develop/blob/c8fa6497aa6dc6d8a65c3fa84b862df343db5981/src/wp-admin/admin.php#L233-L239

[johnbillion]

So, this hook is used by core itself to load the plugin page, but there's technically no reason for it to be ignored / marked as private. Thoughts?

[SergeyBiryukov]

Switching milestone due to the focus on the new editor (Gutenberg) for WordPress 5.0.

[pento]

In 44501:

Docs: Document the $page_hook action.

Props johnbillion.
Fixes #43643.