Add Documentation for FAIR_USER_SCHED sysfs files

This patch adds documentation about /sys/kernel/uids/<uid>/cpu_share
to Documentation/ABI.

Signed-off-by: Dhaval Giani <dhaval@linux.vnet.ibm.com>
Cc: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>