KVM: nVMX: Document 'nested' parameter

Add documentation of the new 'nested' parameter to
'Documentation/kernel-parameters.txt'.

Cc: Avi Kivity <avi@redhat.com>
Cc: Marcelo Tosatti <mtosatti@redhat.com>
Cc: Nadav Har'El <nyh@il.ibm.com>
Signed-off-by: Sasha Levin <levinsasha928@gmail.com>
Signed-off-by: Avi Kivity <avi@redhat.com>