kernel.h: update comment about simple_strto<foo>() functions

kernel.h: update comment about simple_strto<foo>() functions

There were discussions in the past about use cases for
simple_strto<foo>() functions and, in some rare cases, they have a
benefit over kstrto<foo>() ones.

Update a comment to reduce confusion about special use cases.

Link: http://lkml.kernel.org/r/20190801192904.41087-1-andriy.shevchenko@linux.intel.com
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Suggested-by: Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>
Cc: Geert Uytterhoeven <geert+renesas@glider.be>
Cc: Mans Rullgard <mans@mansr.com>
Cc: Petr Mladek <pmladek@suse.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>