Add some clarity to the documentation about Gutenberg requirement

[dartiss]

Update the documentation to clarify that the Gutenberg plugin is required (for now) for the functionality to work.

As discussed on the wp.org forum - https://wordpress.org/support/topic/gutenberg-beta-plugin/

[dartiss]

Although GB will be in core soon, some users on old versions may still be using the plugin, so it will be worth adding that distinction - that those on WordPress 5.0 require nothing else but those on anything less will need to install the Gutenberg plugin as well.