use extra markup to clarify intent

We are specifically talking about the "encoding" keyword here, and the
multipart/form-data media-type (most other multipart types decode to arrays).

Author: karenetheridge

src/oas.md

@@ -1380,10 +1380,10 @@ Array properties MUST be handled by applying the given Encoding Object to produc
 For all other value types for both top-level non-array properties and for values, including array values, within a top-level array, the Encoding Object MUST be applied to the entire value.
 The order of these name-value pairs in the target media type is implementation-defined.
 
-For `application/x-www-form-urlencoded`, the encoding keys MUST map to parameter names, with the values produced according to the rules of the [Encoding Object](#encoding-object).
+For `application/x-www-form-urlencoded`, the `encoding` keys MUST map to parameter names, with the values produced according to the rules of the [Encoding Object](#encoding-object).
 See [Encoding the `x-www-form-urlencoded` Media Type](#encoding-the-x-www-form-urlencoded-media-type) for guidance and examples, both with and without the `encoding` field.
 
-For `multipart`, the encoding keys MUST map to the [`name` parameter](https://www.rfc-editor.org/rfc/rfc7578#section-4.2) of the `Content-Disposition: form-data` header of each part, as is defined for `multipart/form-data` in [[!RFC7578]].
+For `multipart` types that decode to an object, such as `multipart/form-data`, the `encoding` keys MUST map to the [`name` parameter](https://www.rfc-editor.org/rfc/rfc7578#section-4.2) of the `Content-Disposition: form-data` header of each part, as is defined for `multipart/form-data` in [[!RFC7578]].
 See [[!RFC7578]] [Section 5](https://www.rfc-editor.org/rfc/rfc7578.html#section-5) for guidance regarding non-ASCII part names.
 
 See [Encoding `multipart` Media Types](#encoding-multipart-media-types) for further guidance and examples, both with and without the `encoding` field.