Backport of docs: explain access modes for CSI and DHV volumes (#26352) (#26373)

The documentation for CSI and DHV has a list of the available access modes, but
doesn't explain what they mean in terms of what jobs can request, the scheduler
behavior, or the CSI plugin behavior. Expand on the information available in the
CSI specification and provide a description of DHV's behavior as well.

Ref: https://github.com/container-storage-interface/spec/blob/master/spec.md#createvolume

Co-authored-by: Tim Gross <[email protected]>

Author: hc-github-team-nomad-core

website/content/docs/other-specifications/volume/capability.mdx

@@ -40,7 +40,20 @@ available concurrently. Can be one of `"single-node-reader-only"`,
 `"single-node-writer"`, `"multi-node-reader-only"`,
 `"multi-node-single-writer"`, or `"multi-node-multi-writer"`. Most CSI plugins
 support only single-node modes. Consult the documentation of the storage
-provider and CSI plugin.
+provider and CSI plugin. Can be one of the following:
+
+    - `"single-node-reader-only"`: Jobs can only request the volume with
+      read-only access, and only one node can mount the volume at a time.
+    - `"single-node-writer"`: Jobs can request the volume with read/write or
+      read-only access, and only one node can mount the volume at a time.
+    - `"multi-node-reader-only"`: Jobs can only request the volume with
+      read-only access, but multiple nodes can mount the volume simultaneously.
+    - `"multi-node-single-writer"`: Jobs can request the volume with read/write
+      or read-only access, but the scheduler only allows one allocation to have
+      read/write access. Multiple nodes can mount the volume simultaneously.
+    - `"multi-node-multi-writer"`: Jobs can request the volume with read/write
+      or read-only access, and the scheduler allows multiple allocations to have
+      read/write access. Multiple nodes can mount the volume simultaneously.
 
 - `attachment_mode` `(string: <required>)` - The storage API that will be used
 by the volume. Most storage providers will support `"file-system"`, to mount