ipip-0412: clarify car order=unk

#412 (comment)

src/ipips/ipip-0412.md

@@ -85,11 +85,15 @@ as noted in :cite[rfc9110]).
 
 #### `order` CAR content type parameter
 
-The `order` parameter accepts the following values:
+The `order` parameter allows clients to specify the desired block order in the
+response. It supports the following values:
 
 - `dfs`: [Depth-First Search](https://en.wikipedia.org/wiki/Depth-first_search)
-  order, allows for streaming responses with minimal memory usage
-- `rnd`: Unknown (random) order, the implicit default when `order` parameter is missing.
+  order, enables streaming responses with minimal memory usage.
+- `unk`: Unknown order, which serves as the implicit default when the order
+  parameter is missing. In this case, the client cannot make any assumptions
+  about the block order: blocks may arrive in a random order or be a result of
+  a custom DAG traversal algorithm.
 
 #### `dups` CAR content type parameter