init.h: improve __setup and early_param documentation

Igor noted in [1] that there are quite a few __setup() handling
functions that return incorrect values.  Doing this can be harmless, but
it can also cause strings to be added to init's argument or environment
list, polluting them.

Since __setup() handling and return values are not documented, first add
documentation for that.  Also add more documentation for early_param()
handling and return values.

For __setup() functions, returning 0 (not handled) has questionable
value if it is just a malformed option value, as in

  rodata=junk

since returning 0 would just cause "rodata=junk" to be added to init's
environment unnecessarily:

  Run /sbin/init as init process
    with arguments:
      /sbin/init
    with environment:
      HOME=/
      TERM=linux
      splash=native
      rodata=junk

Also, there are no recommendations on whether to print a warning when an
unknown parameter value is seen.  I am not addressing that here.

[1] lore.kernel.org/r/64644a2f-4a20-bab3-1e15-3b2cdd0defe3@omprussia.ru

Link: https://lkml.kernel.org/r/20220221050852.1147-1-rdunlap@infradead.orgSigned-off-by: Randy Dunlap <rdunlap@infradead.org>
Reported-by: Igor Zhbanov <i.zhbanov@omprussia.ru>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/init.h

@@ -320,12 +320,19 @@ struct obs_kernel_param {
 		__aligned(__alignof__(struct obs_kernel_param))		\
 		= { __setup_str_##unique_id, fn, early }
 
+/*
+ * NOTE: __setup functions return values:
+ * @fn returns 1 (or non-zero) if the option argument is "handled"
+ * and returns 0 if the option argument is "not handled".
+ */
 #define __setup(str, fn)						\
 	__setup_param(str, fn, fn, 0)
 
 /*
- * NOTE: fn is as per module_param, not __setup!
- * Emits warning if fn returns non-zero.
+ * NOTE: @fn is as per module_param, not __setup!
+ * I.e., @fn returns 0 for no error or non-zero for error
+ * (possibly @fn returns a -errno value, but it does not matter).
+ * Emits warning if @fn returns non-zero.
  */
 #define early_param(str, fn)						\
 	__setup_param(str, fn, fn, 1)