kernel.h: add comments for enum system_states

kernel.h: add comments for enum system_states

Provide some basic comments about the system_states and what they imply.
Also convert the comments to kernel-doc format.

Split the enum declaration from the definition of the system_state
variable so that kernel-doc notation works cleanly with it.  This is
picked up by Documentation/driver-api/basics.rst so it does not need
further inclusion in the kernel docbooks.

Link: https://lkml.kernel.org/r/20250907043857.2941203-1-rdunlap@infradead.org
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Acked-by: Rafael J. Wysocki <rafael@kernel.org> # v1
Reviewed-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>	[v5]
Cc: "Brown, Len" <len.brown@intel.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: James Bottomley <james.bottomley@HansenPartnership.com>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Mauro Carvalho Chehab <mchehab@kernel.org>
Cc: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>