Document missing usage stats (#39139)

Jordan confirmed that we include iOS, iPadOS, and Android enrollments,
but it's not documented.

<img width="509" height="1050" alt="Screenshot 2026-02-02 at 13 29 42"
src="https://github.com/user-attachments/assets/b5786dd0-7438-4436-8941-7f18fff4014d"
/>

articles/fleet-usage-statistics.md

@@ -44,6 +44,13 @@ Below is the JSON payload that is sent to Fleet Device Management Inc:
   "maintenanceWindowsConfigured": true,
   "numHostsFleetDesktopEnabled": 999,
   "hostsEnrolledByOperatingSystem": {
+    "android": [
+      {
+        "version": "Android 15",
+        "numEnrolled": 999
+      },
+      ...
+    ],
     "darwin": [
       {
         "version": "macOS 12.3.1",
@@ -51,6 +58,20 @@ Below is the JSON payload that is sent to Fleet Device Management Inc:
       },
       ...
     ],
+    "ios": [
+      {
+        "version": "iOS 26.0.1",
+        "numEnrolled": 999
+      },
+      ...
+    ],
+    "ipados": [
+      {
+        "version": "iPadOS 26.1",
+        "numEnrolled": 999
+      },
+      ...
+    ],
     "windows": [
       {
         "version": "Microsoft Windows 10, version 21H2 (W)",