docs: clarify requirements for deleting volumes

If you delete a CSI volume, the volume cannot be currently claimed by an
allocation or in the process of being unpublished. This is documented in the CLI
but not the API. Also, the documentation incorrectly says that the `volume
delete` command silently returns without error if the volume doesn't exist, but
that's incorrect.

Fixes: #24756

Author: tgross

command/volume_delete.go

@@ -25,8 +25,7 @@ Usage: nomad volume delete [options] <vol id>
   Delete a volume from an external storage provider. The volume must still be
   registered with Nomad in order to be deleted. Deleting will fail if the
   volume is still in use by an allocation or in the process of being
-  unpublished. If the volume no longer exists, this command will silently
-  return without an error.
+  unpublished.
 
   When ACLs are enabled, this command requires a token with the
   'csi-write-volume' and 'csi-read-volume' capabilities for the volume's

website/content/api-docs/volumes.mdx

@@ -480,6 +480,10 @@ for deleting the volume as comma-separated key-value pairs (see the
 example below). These secrets will be merged with any secrets already
 stored when the CSI volume was created.
 
+The volume must still be registered with Nomad in order to be deleted. This API
+call fails if an allocation still claims the volume or if Nomad is unpublishing
+the volume.
+
 ### Parameters
 
 - `:volume_id` `(string: <required>)` - Specifies the ID of the

website/content/commands/volume/delete.mdx

@@ -20,10 +20,10 @@ nomad volume delete [options] [volume]
 ```
 
 The `volume delete` command requires a single argument, specifying the ID of
-volume to be deleted. The volume must still be [registered] with Nomad in
-order to be deleted. Deleting will fail if the volume is still in use by an
-allocation or in the process of being unpublished. If the volume no longer
-exists, this command will silently return without an error.
+
+volume to be deleted. The volume must still be [registered][] with Nomad in
+order to be deleted. Deleting fails if the volume is still in use by an
+allocation or in the process of being unpublished.
 
 When ACLs are enabled, this command requires a token with the
 `csi-write-volume` capability for the volume's namespace.