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