swiotlb: update format

Documentation/kernel-parameters.txt doesn't contain up-to-date
documentation regarding swiotlb= parameter. Update it.
Signed-off-by: NJiri Kosina <jkosina@suse.cz>
Signed-off-by: NKonrad Rzeszutek Wilk <konrad.wilk@oracle.com>

Documentation/kernel-parameters.txt

@@ -3061,7 +3061,11 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
 			controller if no parameter or 1 is given or disable
 			it if 0 is given (See Documentation/cgroups/memory.txt)
 
-	swiotlb=	[IA-64] Number of I/O TLB slabs
+	swiotlb=	[ARM,IA-64,PPC,MIPS,X86]
+			Format: { <int> | force }
+			<int> -- Number of I/O TLB slabs
+			force -- force using of bounce buffers even if they
+			         wouldn't be automatically used by the kernel
 
 	switches=	[HW,M68k]