Clarify the rationale for the encoding field

The oddities of its media type support derive from its history
as the OAS implementation of web forms.

Author: handrews

src/oas.md

@@ -1628,7 +1628,7 @@ To use the `encoding` field, a `schema` MUST exist, and the `encoding` field's
 Array properties MUST be handled by applying the given Encoding Object to multiple parts (or query parameters) with the same `name`, as is recommended by [RFC7578](https://www.rfc-editor.org/rfc/rfc7578.html#section-4.3) for supplying multiple values per form field.
 For all other property types, including array values within a top-level array, the Encoding Object MUST be applied to the entire value.
 
-The behavior of the `encoding` field is only defined for media types structured as name-value pairs that allow repeat values.
+The behavior of the `encoding` field is designed to support web forms, and is therefore only defined for media types structured as name-value pairs that allow repeat values.
 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).