doc: kernel-parameters.txt: fix documentation of elevator parameter

Legacy IO schedulers (cfq, deadline and noop) were removed in
f382fb0b.

The documentation for deadline was retained because it carries over to
mq-deadline as well, but location of the doc file was changed over time.

The old iosched algorithms were removed from elevator= kernel parameter
and mq-deadline, kyber and bfq were added with a reference to their
documentation.

Fixes: f382fb0b ("block: remove legacy IO schedulers")
Signed-off-by: Otto Sabart <ottosabart@seberm.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>