printk: improve the description of /dev/kmsg line format

The comment about /dev/kmsg does not mention the additional values which
may actually be exported, fix that.

Also move up the part of the comment instructing the users to ignore these
additional values, this way the reading is more fluent and logically
compact.

Signed-off-by: Antonio Ospite <ao2@ao2.it>
Cc: Joe Perches <joe@perches.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Tejun Heo <tj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>