x86/apic: Update the 'apic=' description of setting APIC driver

There are two consumers of apic=: the APIC debug level and the low
level generic architecture code, but Linux just documented the first
one.

Append the second description.
Signed-off-by: Dou Liyang <douly.fnst@cn.fujitsu.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: peterz@infradead.org
Cc: rdunlap@infradead.org
Cc: corbet@lwn.net
Link: https://lkml.kernel.org/r/20171204040313.24824-2-douly.fnst@cn.fujitsu.com

Documentation/admin-guide/kernel-parameters.txt

@@ -328,11 +328,15 @@
 			not play well with APC CPU idle - disable it if you have
 			APC and your system crashes randomly.
 
-	apic=		[APIC,X86-32] Advanced Programmable Interrupt Controller
+	apic=		[APIC,X86] Advanced Programmable Interrupt Controller
 			Change the output verbosity whilst booting
 			Format: { quiet (default) | verbose | debug }
 			Change the amount of debugging information output
 			when initialising the APIC and IO-APIC components.
+			For X86-32, this can also be used to specify an APIC
+			driver name.
+			Format: apic=driver_name
+			Examples: apic=bigsmp
 
 	apic_extnmi=	[APIC,X86] External NMI delivery setting
 			Format: { bsp (default) | all | none }