Make SP responsible for creation of target_path

In NodePublishVolume, SPs may want to return block devices by either
bind-mounting a device from /dev, or directly calling mknod at the
target path. To allow either approach, make it clear that the SP is
responsible for creation and cleanup for target_path.

csi.proto

@@ -984,7 +984,7 @@ message NodeStageVolumeRequest {
   // is only one `staging_target_path` per volume. The CO SHALL ensure
   // that the path is directory and that the process serving the
   // request has `read` and `write` permission to that directory. The
-  // CO SHALL be responsible for creating the path if it does not
+  // CO SHALL be responsible for creating the directory if it does not
   // exist.
   // This is a REQUIRED field.
   string staging_target_path = 3;
@@ -1044,11 +1044,14 @@ message NodePublishVolumeRequest {
   // The path to which the volume will be published. It MUST be an
   // absolute path in the root filesystem of the process serving this
   // request. The CO SHALL ensure uniqueness of target_path per volume.
-  // The CO SHALL ensure that the path exists and that the process
-  // serving the request has `read` and `write` permissions to the path.
-  // For volumes with an access type of block, the path MUST be a file.
-  // For volumes with an access type of mount, the path MUST be a
-  // directory. The SP SHALL bind mount the volume to the target path.
+  // The CO SHALL ensure that the parent directory of this path exists
+  // and that the process serving the request has `read` and `write`
+  // permissions to that parent directory.
+  // For volumes with an access type of block, the SP SHALL place the
+  // block device at target_path.
+  // For volumes with an access type of mount, the SP SHALL place the
+  // mounted directory at target_path.
+  // Creation of target_path is the responsibility of the SP.
   // This is a REQUIRED field.
   string target_path = 4;
 
@@ -1082,6 +1085,7 @@ message NodeUnpublishVolumeRequest {
 
   // The path at which the volume was published. It MUST be an absolute
   // path in the root filesystem of the process serving this request.
+  // The SP MUST delete this path.
   // This is a REQUIRED field.
   string target_path = 2;
 }