docs: cgroup-v2: fix stale "io" controller introduction

The introductory paragraph for the IO controller still states that
weight based distribution is "available only if cfq-iosched is in use"
and that "neither scheme is available for blk-mq devices".  This text
dates from when the cgroup v2 documentation was first written (2015)
and was correct at the time, but is no longer accurate:

  * cfq-iosched was removed in v5.0;
  * blk-mq is now the only block I/O path, and both the absolute limit
    scheme (io.max via blk-throttle) and the weight based scheme
    (io.weight via iocost, or io.bfq.weight under BFQ) work on it;
  * latency based protection (iolatency) and I/O priority (ioprio)
    controllers have since been added.

The rest of the section already documents io.weight, io.max,
io.cost.{qos,model}, io.latency and io.prio.class correctly, so the
introduction is the only part that contradicts them.  Rewrite it to
reflect the current state.

Signed-off-by: Zhe Liu <liuzhe1@kylinos.cn>
Reviewed-by: Tao Cui <cuitao@kylinos.cn>
Signed-off-by: Tejun Heo <tj@kernel.org>

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