docs: submitting-patches: Emphasise the requirement to Cc: stable when using Fixes...

docs: submitting-patches: Emphasise the requirement to Cc: stable when using Fixes: tag

Clear-up any confusion surrounding the Fixes: tag with regards to the
need to Cc: the stable mailing list when submitting stable patch
candidates.

Signed-off-by: Lee Jones <lee.jones@linaro.org>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: linux-doc@vger.kernel.org
Link: https://lore.kernel.org/r/20210113163315.1331064-1-lee.jones@linaro.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>