Show private banner for class methods that are private

[coffee2code]

The current banner that displays for private functions does not display for private class methods not explicitly documented with @access private. This is due to two things:

1. get_private_access_message() does not handle wp-parser-method posts (it needs to be whitelisted)
2. Even once get_private_access_message() handles the post type, the function needs to fall back to checking the post's '_wp-parser_visibility' meta field to see if it was defined with an access modifier of 'private'.

Example: ​https://developer.wordpress.org/reference/classes/_wp_editors/get_translation/

[coffee2code]

In 4580:

developer.wordpress.org: Enable private message notice for class methods.

See #2344.

[coffee2code]

In 4581:

developer.wordpress.org: Also check access modifier to determine if function or class method is private.

Fixes #2344.