spec: Clarify calling multiple NodePublishVolume

Clarifies that `NodePublishVolume` can only be called multiple times on
a node for a volume if that volume can be published on multiple nodes
(i.e., has MULTI_NODE capability).

If a volume cannot be published on multiple node, it's weird that we
allow it to be publish multiple times on a single node. In other words,
it's weird that we allow two workloads to share a volume if they are on
the same node, but don't have a way to do that if they are on different
nodes.

Author: jieyu

spec.md

@@ -998,7 +998,7 @@ If the plugin is unable to complete the ControllerGetCapabilities call successfu
 
 This RPC is called by the CO when a workload that wants to use the specified volume is placed (scheduled) on a node.
 The Plugin SHALL assume that this RPC will be executed on the node where the volume will be used.
-This RPC MAY be called by the CO multiple times on the same node for the same volume with possibly different `target_path` and/or auth credentials.
+This RPC MAY be called by the CO multiple times on the same node for the same volume with possibly different `target_path` and/or auth credentials if the volume has MULTI_NODE capability (i.e., `volume_capability` is either `MULTI_NODE_READER_ONLY`, `MULTI_NODE_SINGLE_WRITER` or `MULTI_NODE_MULTI_WRITER`).
 If the corresponding Controller Plugin has `PUBLISH_UNPUBLISH_VOLUME` controller capability, the CO MUST guarantee that this RPC is called after `ControllerPublishVolume` is called for the given volume on the given node and returns a success.
 
 This operation MUST be idempotent.