docs: kernel-parameter: Improve the description of nr_cpus and maxcpus

From the old description people still can't get what's the exact
difference between nr_cpus and maxcpus. Especially in kdump kernel
nr_cpus is always suggested if it's implemented in the ARCH. The
reason is nr_cpus is used to limit the max number of possible cpu
in system, the sum of already plugged cpus and hot plug cpus can't
exceed its value. However maxcpus is used to limit how many cpus
are allowed to be brought up during bootup.

Signed-off-by: Baoquan He <bhe@redhat.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>