doc: make doc. for maxcpus= more visible

Some people are confused about maxcpus=1 and maxcpus=0,
so put the documentation text from init/main.c into
Documentation/kernel-parameters.txt also.

Signed-off-by: Randy Dunlap <rdunlap@xenotime.net>
Signed-off-by: Adrian Bunk <bunk@stusta.de>