Documentation: update /proc/uptime field description

/proc/uptime has two fields, update description to describe both
ala http://lkml.indiana.edu/hypermail/linux/kernel/1312.3/01454.html



Signed-off-by: Rob Landley <rob@landley.net>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>