Explicitly say `count` is not supported by modules (#10553)

Reference: https://groups.google.com/forum/#!msg/terraform-tool/T3eB4pZ6waU/5hqs2SyGCAAJ

website/source/docs/configuration/resources.html.md

@@ -50,6 +50,8 @@ There are **meta-parameters** available to all resources:
       This doesn't apply to all resources. For details on using variables in
       conjunction with count, see [Using Variables with
      `count`](#using-variables-with-count) below.
+     ~> **NOTE:** modules don't currently support the `count` parameter.
+
 
   * `depends_on` (list of strings) - Explicit dependencies that this
       resource has. These dependencies will be created before this