Clarify lack of nested multipart support.

Author: handrews

src/oas.md

@@ -1583,6 +1583,8 @@ Determining how to handle a `type` value of `null` depends on how `null` values
 If `null` values are entirely omitted, then the `contentType` is irrelevant.
 See [Appendix B](#appendix-b-data-type-conversion) for a discussion of data type conversion options.
 
+It is not currently possible to model nested `multipart` media types.
+
 ###### Fixed Fields for RFC6570-style Serialization
 
 | Field Name | Type | Description |