Documentation/process/howto: Only send regression fixes after -rc1

The original text was not clear if white space or other harmless patches
should be merged in -rc kernels.  The discussion at Kernel Summit said
that we should be more strict about sending regression fixes only.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>