docs: CSI mount_options are available only for filesystem vols (#9043)

The CSI specification allows only the `file-system` attachment mode to have
mount options. The `block-device` mode is left "intentionally empty, for now"
in the protocol. We should be validating against this problem, but our
documentation also had it backwards.

Also adds missing mount_options on group volume.

website/pages/docs/commands/volume/register.mdx

@@ -82,14 +82,16 @@ context {
   single-node modes. Consult the documentation of the storage provider
   and CSI plugin.
 
-- `attachment_mode` `(string: <required>)` - The storage API that will
-  be used by the volume. Most storage providers will support
-  `"file-system"`, to mount pre-formatted file system volumes. Some
-  storage providers will support `"block-device"`, which will require
-  the job be configured with appropriate mount options.
-
-- `mount_options` <code>([mount_options][]:nil)</code> - Options for
-  mounting `block-device`volumes without a pre-formatted file system.
+- `attachment_mode` `(string: <required>)` - The storage API that will be used
+  by the volume. Most storage providers will support `"file-system"`, to mount
+  volumes using the CSI filesystem API. Some storage providers will support
+  `"block-device"`, which will mount the volume with the CSI block device API
+  within the container.
+
+- `mount_options` - Options for mounting `file-system` volumes that don't
+  already have a pre-formatted file system. Consult the documentation for your
+  storage provider and CSI plugin as to whether these options are required or
+  neccessary.
 
   - `fs_type`: file system type (ex. `"ext4"`)
   - `mount_flags`: the flags passed to `mount` (ex. `"ro,noatime"`)

website/pages/docs/job-specification/volume.mdx

@@ -55,7 +55,20 @@ the [volume_mount][volume_mount] stanza in the `task` configuration.
   used for validating `host_volume` ACLs and for scheduling when a
   matching `host_volume` requires `read_only` usage.
 
+- `mount_options` - Options for mounting CSI volumes that have the
+  `file-system` [attachment mode]. These options override the `mount_options`
+  field from [volume registration]. Consult the documentation for your storage
+  provider and CSI plugin as to whether these options are required or
+  neccessary.
+
+  - `fs_type`: file system type (ex. `"ext4"`)
+  - `mount_flags`: the flags passed to `mount` (ex. `"ro,noatime"`)
+
+
 [volume_mount]: /docs/job-specification/volume_mount 'Nomad volume_mount Job Specification'
 [host_volume]: /docs/configuration/client#host_volume-stanza
 [csi_volume]: /docs/commands/volume/register
 [csi_plugin]: /docs/job-specification/csi_plugin
+[csi_volume]: /docs/commands/volume/register
+[attachment mode]: /docs/commands/volume/register#attachment_mode
+[volume registration]: /docs/commands/volume/register#mount_options