docs: provide guidance on setting environment variables for NLA (#59938)

We often get support requests for help on setting environment variables,
particularly in regards to enabling NLA for Windows desktop access.

Provide some high level guidance for both Kuberenetes and systemd.

Closes #59731

Author: zmb3

docs/pages/enroll-resources/desktop-access/active-directory.mdx

@@ -641,7 +641,7 @@ To configure Teleport to protect access to Windows desktops:
    windows_desktop_service:
       enabled: true
       ldap:
-         locate_server: 
+         locate_server:
             enabled: true
             site: "my-site" # optional
          domain: example.com
@@ -865,10 +865,33 @@ windows_desktop_service:
 ```
 
 To enable NLA, set the `TELEPORT_ENABLE_RDP_NLA` environment variable to `yes`
-on any hosts running the Teleport's `windows_desktop_service`. Note that NLA is
-only supported when connecting to hosts that are part of an Active Directory
-domain. Teleport will not perform NLA when connecting to hosts as a local
-Windows user.
+on any hosts running the Teleport's `windows_desktop_service`. The process for
+setting an environment variable varies depending on the environment in which you
+are running Teleport.
+
+If you're running Teleport in Kubernetes, you'll need to
+[edit the Pod configuration](https://kubernetes.io/docs/tasks/inject-data-application/define-environment-variable-container/).
+
+If you're running Teleport as a systemd service, then you'll want to create a
+systemd override using `systemctl edit teleport`:
+
+```
+$ sudo systemctl edit teleport
+```
+
+This will open a text editor where you can insert an override for the environment:
+
+```
+### Editing /etc/systemd/system/teleport.service.d/override.conf
+### Anything between here and the comment below will become the contents of the drop-in file
+
+[Service]
+Environment="TELEPORT_ENABLE_RDP_NLA=yes"
+```
+
+Note that NLA is only supported when connecting to hosts that are part of an
+Active Directory domain. Teleport will not perform NLA when connecting to hosts
+as a local Windows user.
 
 NLA is not supported when Teleport runs in FIPS mode.