describe ABORTED as error message for CreateVolumeGroupSnapshot

Just like ABORTED is an error for the CreateSnapshot procedure, it
should be documented for CreateVolumeGroupSnapshot.

spec.md

@@ -3051,6 +3051,7 @@ The CO MUST implement the specified error recovery behavior when it encounters t
 |-----------|-----------|-------------|-------------------|
 | Group snapshot already exists but is incompatible | 6 ALREADY_EXISTS | Indicates that a group snapshot corresponding to the specified group snapshot `name` already exists but is incompatible with the specified `source_volume_ids` or `parameters`. | Caller MUST fix the arguments or use a different `name` before retrying. |
 | Cannot snapshot multiple volumes together | 9 FAILED_PRECONDITION | Indicates that the specified volumes cannot be snapshotted together because the volumes are not configured properly based on requirements from the SP. | Caller MUST fix the configuration of the volumes so that they meet the requirements for group snapshotting before retrying. |
+| Operation pending for group snapshot | 10 ABORTED | Indicates that there is already an operation pending for the a group snapshot with the exact same parameters. In general the CO is responsible for ensuring that there is no more than one call "in-flight" per group snapshot at a given time. However, in some circumstances, the CO MAY lose state (for example when the CO crashes and restarts), and MAY issue multiple calls simultaneously for the same group snapshot. The Plugin, SHOULD handle this as gracefully as possible, and MAY return this error code to reject secondary calls. | Caller SHOULD ensure that there are no other calls pending for the same group snapshot, and then retry with exponential back off. |
 | Not enough space to create group snapshot | 13 RESOURCE_EXHAUSTED | There is not enough space on the storage system to handle the create group snapshot request. | Future calls to CreateVolumeGroupSnapshot MAY succeed if space is freed up. |
 
 #### `DeleteVolumeGroupSnapshot`