docs: process: submitting-patches: Clarify the Reported-by usage

It's unclear from "Submitting Patches" documentation that Reported-by
is not supposed to be used against new features. (It's more clear
in the section 5.4 "Patch formatting and changelogs" of the "A guide
to the Kernel Development Process", where it suggests that change
should fix something existing in the kernel. Clarify the Reported-by
usage in the "Submitting Patches".

Reported-by: Florian Eckert <fe@dev.tdt.de>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Link: https://lore.kernel.org/r/20220127163258.48482-1-andriy.shevchenko@linux.intel.com


Signed-off-by: Jonathan Corbet <corbet@lwn.net>