Mention about the possibilities of using staging_target_path at NodeS…

…tage.

At present, it is unclear to SP that, how to use staging_target_path
and the possibilities of using the same. This patch add more details
on `staging target path` and its usage.

Fix # #385

Signed-off-by: Humble Chirammal <[email protected]>

spec.md

@@ -1931,7 +1931,12 @@ message NodeStageVolumeRequest {
   // 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 directory if it does not
-  // exist.
+  // exist.The CO will pass around the staging_target_directory from
+  // NodeStageVolume to NodePublishVolume. SP can stage a volume
+  // however it like inside the staging_target_directory.
+  // For example, SP can stage it directly at the starging_target_directory,
+  // or a subdirectory or file underneath. SP can also store other files/metadata
+  // inside the staging_target_directory if it like.
   // This is a REQUIRED field.
   string staging_target_path = 3;