cxl/core: Improve CXL core kernel docs

/**

 * DOC: cxl core

 *

 * The CXL core provides a sysfs hierarchy for control devices and a rendezvous

 * point for cross-device interleave coordination through cxl ports.

 * The CXL core provides a set of interfaces that can be consumed by CXL aware

 * drivers. The interfaces allow for creation, modification, and destruction of

 * regions, memory devices, ports, and decoders. CXL aware drivers must register

 * with the CXL core via these interfaces in order to be able to participate in

 * cross-device interleave coordination. The CXL core also establishes and

 * maintains the bridge to the nvdimm subsystem.

 *

 * CXL core introduces sysfs hierarchy to control the devices that are

 * instantiated by the core.

 */

static DEFINE_IDA(cxl_port_ida);