doc/hotplug: update man and --help

Update the man page and --help option to make the description of the
--hotplug option easier to understand.

Acked-by: Baoquan He <bhe@redhat.com>
Acked-by: Hari Bathini <hbathini@linux.ibm.com>
Signed-off-by: Sourabh Jain <sourabhjain@linux.ibm.com>
Signed-off-by: Simon Horman <horms@kernel.org>