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>

Documentation/admin-guide/kernel-parameters.txt

@@ -1182,9 +1182,10 @@
 			arch/x86/kernel/cpu/cpufreq/elanfreq.c.
 
 	elevator=	[IOSCHED]
-			Format: {"cfq" | "deadline" | "noop"}
-			See Documentation/block/cfq-iosched.txt and
-			Documentation/block/deadline-iosched.txt for details.
+			Format: { "mq-deadline" | "kyber" | "bfq" }
+			See Documentation/block/deadline-iosched.txt,
+			Documentation/block/kyber-iosched.txt and
+			Documentation/block/bfq-iosched.txt for details.
 
 	elfcorehdr=[size[KMG]@]offset[KMG] [IA64,PPC,SH,X86,S390]
 			Specifies physical address of start of kernel core