Document main multipart array change

Author: dbanty

.changeset/repeat_array_fields_in_multipart_instead_of_serializing_as_json.md

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+---
+default: major
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+# Change default multipart array serialization
+
+Previously, any arrays of values in a `multipart/form-data` body would be serialized as an `application/json` part.
+This matches the default behavior specified by OpenAPI and supports arrays of files (`binary` format strings).
+However, because this generator doesn't yet support specifying `encoding` per property, this may result in 
+now-incorrect code when the encoding _was_ explicitly set to `application/json` for arrays of scalar values.
+
+PR #938 fixes #692. Thanks @micha91 for the fix, @ratgen and @FabianSchurig for testing, and @davidlizeng for the original report... many years ago 😅.