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>

Author: rddunlap

include/linux/kernel.h

@@ -164,11 +164,23 @@ extern int root_mountflags;
 
 extern bool early_boot_irqs_disabled;
 
-/*
- * Values used for system_state. Ordering of the states must not be changed
+/**
+ * enum system_states - Values used for system_state.
+ *
+ * @SYSTEM_BOOTING:	%0, no init needed
+ * @SYSTEM_SCHEDULING: system is ready for scheduling; OK to use RCU
+ * @SYSTEM_FREEING_INITMEM: system is freeing all of initmem; almost running
+ * @SYSTEM_RUNNING:	system is up and running
+ * @SYSTEM_HALT:	system entered clean system halt state
+ * @SYSTEM_POWER_OFF:	system entered shutdown/clean power off state
+ * @SYSTEM_RESTART:	system entered emergency power off or normal restart
+ * @SYSTEM_SUSPEND:	system entered suspend or hibernate state
+ *
+ * Note:
+ * Ordering of the states must not be changed
  * as code checks for <, <=, >, >= STATE.
  */
-extern enum system_states {
+enum system_states {
 	SYSTEM_BOOTING,
 	SYSTEM_SCHEDULING,
 	SYSTEM_FREEING_INITMEM,
@@ -177,7 +189,8 @@ extern enum system_states {
 	SYSTEM_POWER_OFF,
 	SYSTEM_RESTART,
 	SYSTEM_SUSPEND,
-} system_state;
+};
+extern enum system_states system_state;
 
 /*
  * General tracing related utility functions - trace_printk(),