device property: Fix documentation for FWNODE_GRAPH_DEVICE_DISABLED

FWNODE_GRAPH_DEVICE_DISABLED flag was meant for also returning endpoints
connected to disabled devices, but it also may return endpoints that are
not connected. Fix this in documentation. Also
fwnode_graph_get_endpoint_by_id() was affeced by this.

Also improve the language a little bit.

Fixes: 0fcc2bdc ("device property: Add fwnode_graph_get_endpoint_by_id()")
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>