Add additional note on CRT auto mode

The CRT is_optimized_for_system() utility only returns True
for environments running Linux. So this update clarifies that
the system must be running Linux in addition to being one of the
enumerated instance types in order to auto resolve into using
the CRT client.

awscli/topics/s3-config.rst

@@ -283,7 +283,8 @@ files to and from S3. Valid choices are:
 
   * The host running the AWS CLI is optimized for the ``crt`` transfer client.
     Currently, the ``crt`` transfer client is optimized for Amazon EC2 instances
-    that are of any of these instance types:
+    that are running Linux as the operating system and are of any of these
+    instance types:
 
     * ``p4d.24xlarge``
     * ``p4de.24xlarge``