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>

Documentation/ABI/testing/sysfs-kernel-uids

+What:		/sys/kernel/uids/<uid>/cpu_shares
+Date:		December 2007
+Contact:	Dhaval Giani <dhaval@linux.vnet.ibm.com>
+		Srivatsa Vaddagiri <vatsa@linux.vnet.ibm.com>
+Description:
+		The /sys/kernel/uids/<uid>/cpu_shares tunable is used
+		to set the cpu bandwidth a user is allowed. This is a
+		propotional value. What that means is that if there
+		are two users logged in, each with an equal number of
+		shares, then they will get equal CPU bandwidth. Another
+		example would be, if User A has shares = 1024 and user
+		B has shares = 2048, User B will get twice the CPU
+		bandwidth user A will. For more details refer
+		Documentation/sched-design-CFS.txt