CSHARP-3448: Update FLE documentation to mention that we currently don't support arm architecture. (#481)

Author: DmitryLukyanov

Docs/reference/content/reference/driver/crud/client_side_encryption.md

@@ -21,6 +21,10 @@ field level encryption supports workloads where applications must guarantee that
 unauthorized parties, including server administrators, cannot read the encrypted
 data.
 
+{{% note class="important" %}} 
+Client-side field level encryption in the .NET/C# driver is currently only supported on x64-compatible CPUs.
+{{% /note %}}
+
 ## mongocryptd configuration
 
 Client-side field level encryption requires the `mongocryptd` daemon / process