include parallel clause for encoding by position

This is the same phrasing as in "Encoding by Name" which states that the
`encoding` fields do nothing if they don't have a corresponding data
instance.

Author: karenetheridge

src/oas.md

@@ -1396,7 +1396,8 @@ Data for these media types are modeled as an array, with one item per part, in o
 
 For applications that wish to preserve part order, `multipart/form-data` content may also be modelled as an array, with one item per part, in order (where each item consists of an object containing the name/value pair); the `schema` SHALL be used to determine whether deserializing to an `object` or an `array` is preferred. See examples in [Media Type Registry: Forms](https://spec.openapis.org/registry/media-type/forms).
 
-To use the `prefixEncoding` and/or `itemEncoding` fields, either `itemSchema` or a [`schema` indicating an array type](#non-json-data) MUST be present.
+To use the `prefixEncoding` and/or `itemEncoding` fields, each item referenced by the field MUST exist in the data instance; `prefixEncoding` and `itemEncoding` entries with no corresponding item SHALL be ignored.
+
 These fields are analogous to the `prefixItems` and `items` JSON Schema keywords, with `prefixEncoding` (if present) providing an array of Encoding Objects that are each applied to the value at the same position in the data array, and `itemEncoding` applying its single Encoding Object to all remaining items in the array.
 As with `prefixItems`, it is _not_ an error if the instance array is shorter than the `prefixEncoding` array; the additional Encoding Objects SHALL be ignored.