Add notes for /srv/node in swift-object-info

'devices' is set in object-server.conf on each node, not in ring data,
and the things printed here is just for watching not for running, so
just leave a note here. (this https://review.openstack.org/#/c/23951/
is used for running, so just a note is not enough)

mark this commit as bug fixing is because this script is the last place
using /srv/node but not from conf as Chmouel said.

fixes import change on read_metadata
fixes bug #885006
Change-Id: I727ec2d01c093af61fd3895e5701d87ef67cd9ff
Merge "Add notes for /srv/node in swift-object-info"