api-ref: fix instance action 'message' description
api-ref: fix instance action 'message' description

There was a typo in the description of this parameter
and it also didn't read very clearly, so I've just
re-written the description of this.

On a side note, it's a bit odd that this is a required
parameter in the response (Tempest requires it in the
response schema) even though it's only set if the
action failed.

Change-Id: Ic84031119e2ffb6b1fb0b0a193967b2823464651

gooddata/openstack-nova,jianghuaw/nova,vmturbo/nova,klmitch/nova,mikalstill/nova,gooddata/openstack-nova,Juniper/nova,mikalstill/nova,Juniper/nova,gooddata/openstack-nova,mahak/nova,openstack/nova,phenoxim/nova,rajalokan/nova,mahak/nova,rajalokan/nova,rahulunair/nova,rajalokan/nova,rajalokan/nova,openstack/nova,gooddata/openstack-nova,vmturbo/nova,mikalstill/nova,vmturbo/nova,rahulunair/nova,jianghuaw/nova,vmturbo/nova,klmitch/nova,Juniper/nova,phenoxim/nova,klmitch/nova,Juniper/nova,jianghuaw/nova,jianghuaw/nova,openstack/nova,mahak/nova,klmitch/nova,rahulunair/nova