KAFKA-12570: Document compact arrays in flexible versions

Generated-by: OpenAI Codex

Author: gabrieldutra

docs/design/protocol.md

@@ -136,6 +136,8 @@ For interoperability with 0.9.0.x clients, the first packet received by the serv
 
 The protocol is built out of the following primitive types.
 
+Message versions marked as flexible use the compact encoding for variable-length fields. In particular, arrays use `COMPACT_ARRAY` rather than `ARRAY`; strings and byte arrays similarly use their corresponding `COMPACT_*` types. Compact encodings store their length as an unsigned variable-length integer instead of a fixed-width integer. Flexible versions also include a tagged-fields section at the end of every request and response component.
+
 {{< include-html file="/static/{version}/generated/protocol_types.html" >}} 
 
 ### Notes on reading the request format grammars