UPSTREAM: Documentation/sysctl: Document uclamp sysctl knobs

Uclamp exposes 3 sysctl knobs:

	* sched_util_clamp_min
	* sched_util_clamp_max
	* sched_util_clamp_min_rt_default

Document them in sysctl/kernel.rst.

Signed-off-by: Qais Yousef <qais.yousef@arm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20200716110347.19553-3-qais.yousef@arm.com


(cherry picked from commit 1f73d1ab)
Signed-off-by: Qais Yousef <qais.yousef@arm.com>
Change-Id: Ia62d32866ae9d26db9b7d15517d017fea0c315b0