Loosen up wording for OFFLINE expansion

[gnufied]

Sometimes a CO may not accurately be able to verify if volume is already published to a node or not when calling ControllerExpandVolume RPC call and in that case an ControllerExpandVolume call might be issued to the driver even though driver only supports offline expansion and underlying is in-use.

Currently there is a explicit error condition to report when this happens:

| Volume in use | 9 FAILED_PRECONDITION | Indicates that the volume corresponding to the 
specified `volume_id` could not be expanded because it is currently published on a node but the 
plugin does not have ONLINE expansion capability. | Caller SHOULD ensure that volume is not 
published and retry with exponential back off. |

However the wording of OFFLINE only expansion uses following language:

OFFLINE indicates that volumes currently published and available on a node SHALL NOT be 
expanded via ControllerExpandVolume

Which is kinda problematic.