ACPI: docs: enumeration: Clarify ACPI bus concepts

In some cases, ACPI drivers are implemented as a way to manage devices
enumerated with the help of the platform firmware through ACPI.

This might be confusing, since the preferred way to implement a driver
for a device that cannot be enumerated natively, is a platform
driver, as stated in the documentation.

Clarify relationships between ACPI device objects, platform devices and
ACPI Namespace entries.

Suggested-by: Elena Reshetova <elena.reshetova@intel.com>
Co-developed-by: Michal Wilczynski <michal.wilczynski@intel.com>
Signed-off-by: Michal Wilczynski <michal.wilczynski@intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>