Documentation: document the nosmt and smt s390 kernel parameters

Commit 6c2ff84e979a ("s390: add SMT support") added the nosmt and smt
early kernel parameters for restricting the use of symmetric
multithreading (SMT). They are quite useful for debugging and testing,
so document them.
Signed-off-by: Sascha Silbe <silbe@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

Documentation/kernel-parameters.txt

@@ -2538,6 +2538,9 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
 
 	nohugeiomap	[KNL,x86] Disable kernel huge I/O mappings.
 
+	nosmt		[KNL,S390] Disable symmetric multithreading (SMT).
+			Equivalent to smt=1.
+
 	noxsave		[BUGS=X86] Disables x86 extended register state save
 			and restore using xsave. The kernel will fallback to
 			enabling legacy floating-point and sse state.
@@ -3695,6 +3698,13 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
 				1: Fast pin select (default)
 				2: ATC IRMode
 
+	smt		[KNL,S390] Set the maximum number of threads (logical
+			CPUs) to use per physical CPU on systems capable of
+			symmetric multithreading (SMT). Will be capped to the
+			actual hardware limit.
+			Format: <integer>
+			Default: -1 (no limit)
+
 	softlockup_panic=
 			[KNL] Should the soft-lockup detector generate panics.
 			Format: <integer>