docs: clarify data dir default parameters and default creation.

The data dir parameters for client and server agents confusingly
detailed string literal values which were intended to describe
interpolated values. This change attempts to remove the confusion
and clarify the wording slightly.

website/content/docs/configuration/client.mdx

@@ -22,10 +22,10 @@ client {
 
 ## `client` Parameters
 
-- `alloc_dir` `(string: "[data_dir]/alloc")` - Specifies the directory to use
-  for allocation data. By default, this is the top-level
-  [data_dir](/nomad/docs/configuration#data_dir) suffixed with
-  "alloc", like `"/opt/nomad/alloc"`. This must be an absolute path.
+- `alloc_dir` `(string: "")` - Specifies the directory to use for allocation
+  data. When this parameter is empty, Nomad will generate the path using the
+  [top-level data_dir][top_level_data_dir] suffixed with "alloc", like
+  `"/opt/nomad/alloc"`. This must be an absolute path.
 
 - `chroot_env` <code>([ChrootEnv](#chroot_env-parameters): nil)</code> -
   Specifies a key-value mapping that defines the chroot environment for jobs
@@ -115,10 +115,10 @@ client {
   the server address or use go-discover syntax for auto-discovery. See the
   documentation for more detail.
 
-- `state_dir` `(string: "[data_dir]/client")` - Specifies the directory to use
-  to store client state. By default, this is - the top-level
-  [data_dir](/nomad/docs/configuration#data_dir) suffixed with
-  "client", like `"/opt/nomad/client"`. This must be an absolute path.
+- `state_dir` `(string: "")` - Specifies the directory to use to store client
+  state. When this parameter is empty, Nomad will generate the path using the
+  [top-level data_dir][top_level_data_dir] suffixed with "client", like
+  `"/opt/nomad/client"`. This must be an absolute path.
 
 - `gc_interval` `(string: "1m")` - Specifies the interval at which Nomad
   attempts to garbage collect terminal allocation directories.
@@ -744,3 +744,4 @@ client {
 [migrate]: /nomad/docs/job-specification/migrate
 [`nomad node drain -self -no-deadline`]: /nomad/docs/commands/node/drain
 [`TimeoutStopSec`]: https://www.freedesktop.org/software/systemd/man/systemd.service.html#TimeoutStopSec=
+[top_level_data_dir]: /nomad/docs/configuration#data_dir

website/content/docs/configuration/index.mdx

@@ -276,10 +276,10 @@ testing.
   value is used to identify individual agents. When specified on a server, the
   name must be unique within the region.
 
-- `plugin_dir` `(string: "[data_dir]/plugins")` - Specifies the directory to
-  use for looking up plugins. By default, this is the top-level
-  [data_dir](#data_dir) suffixed with "plugins", like `"/opt/nomad/plugins"`.
-  This must be an absolute path.
+- `plugin_dir` `(string: "")` - Specifies the directory to use for looking up
+  plugins. When this parameter is empty, Nomad will generate the path using the
+  top-level [data_dir](#data_dir) suffixed with "plugins", like
+  `"/opt/nomad/plugins"`. This must be an absolute path.
 
 - `plugin` `(`[`Plugin`]`: nil)` - Specifies configuration for a
   specific plugin. The plugin block may be repeated, once for each plugin being

website/content/docs/configuration/server.mdx

@@ -43,11 +43,13 @@ server {
   `1` does not provide any fault tolerance and is not recommended for production
   use cases.
 
-- `data_dir` `(string: "[data_dir]/server")` - Specifies the directory to use
-  for server-specific data, including the replicated log. By default, this is
-  the top-level [data_dir](/nomad/docs/configuration#data_dir) suffixed with "server",
-  like `"/opt/nomad/server"`. The top-level option must be set, even when
-  setting this value. This must be an absolute path.
+- `data_dir` `(string: "")` - Specifies the directory to use for server-specific
+  data, including the replicated log. When this parameter is empty, Nomad will
+  generate the path using the [top-level data_dir][top_level_data_dir] suffixed
+  with "server", like `"/opt/nomad/server"`. The
+  [top-level data_dir][top_level_data_dir] must be set, even when setting this
+  value. This must be an absolute path. Nomad will create the directory on the
+  host, if it does not exist when the agent process starts.
 
 - `enabled` `(bool: false)` - Specifies if this agent should run in server mode.
   All other server options depend on this value being set.
@@ -516,3 +518,4 @@ work.
 [herd]: https://en.wikipedia.org/wiki/Thundering_herd_problem
 [wi]: /nomad/docs/concepts/workload-identity
 [Configure for multiple regions]: /nomad/tutorials/access-control/access-control-bootstrap#configure-for-multiple-regions
+[top_level_data_dir]: /nomad/docs/configuration#data_dir