doc: clarify port hotplug in testpmd
doc: clarify port hotplug in testpmd

Port needs to be stopped and then closed before it can be detached,
but the documentation was only saying to close the port.

Also, both sections for port detaching and attaching has been reformatted
slightly, to show clearly how to use the commands.

Signed-off-by: Pablo de Lara <28355d9cfe41bb8148aea0fb24f47aae8f4217a1@intel.com>
Acked-by: John McNamara <af31595c36a4239b9676e9b3a777d3e2f15a6c4d@intel.com>

john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk