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>