Documentation/kernel-parameters.txt: update 'isolcpus' kernel option

cpuset can be used to move a process onto or off an isolated CPU.

Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>