init.h: add some more documentation to __ref* tags

The __ref* tags may have been confusing for new kernel
developers (I was confused by them for sure) so adding a few
more sentences to comment to clear things up for people who
see those for the first time.
Signed-off-by: Michal Nazarewicz <m.nazarewicz@samsung.com>
Acked-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

include/linux/init.h

@@ -46,16 +46,23 @@
 #define __exitdata	__section(.exit.data)
 #define __exit_call	__used __section(.exitcall.exit)
 
-/* modpost check for section mismatches during the kernel build.
+/*
+ * modpost check for section mismatches during the kernel build.
  * A section mismatch happens when there are references from a
  * code or data section to an init section (both code or data).
  * The init sections are (for most archs) discarded by the kernel
  * when early init has completed so all such references are potential bugs.
  * For exit sections the same issue exists.
+ *
  * The following markers are used for the cases where the reference to
  * the *init / *exit section (code or data) is valid and will teach
- * modpost not to issue a warning.
- * The markers follow same syntax rules as __init / __initdata. */
+ * modpost not to issue a warning.  Intended semantics is that a code or
+ * data tagged __ref* can reference code or data from init section without
+ * producing a warning (of course, no warning does not mean code is
+ * correct, so optimally document why the __ref is needed and why it's OK).
+ *
+ * The markers follow same syntax rules as __init / __initdata.
+ */
 #define __ref            __section(.ref.text) noinline
 #define __refdata        __section(.ref.data)
 #define __refconst       __section(.ref.rodata)