Make check_visibility() a documented API

[simonw]

Spotted this while working on:

• Remove check_permission() from BaseView #1677

datasette/datasette/utils/__init__.py

Lines 1005 to 1021 in e627510

	async def check_visibility(datasette, actor, action, resource, default=True):
	"""Returns (visible, private) - visible = can you see it, private = can others see it too"""
	visible = await datasette.permission_allowed(
	actor,
	action,
	resource=resource,
	default=default,
	)
	if not visible:
	return False, False
	private = not await datasette.permission_allowed(
	None,
	action,
	resource=resource,
	default=default,
	)
	return visible, private

[simonw]

Documentation: https://docs.datasette.io/en/latest/internals.html#await-check-visibility-actor-action-resource-none