Document rewind behavior change of rack input.

UPGRADE-GUIDE.md

@@ -178,7 +178,13 @@ dropped entirely.
 Previously, `rack.input` was required to be rewindable, i.e. `io.seek(0)` but
 this was only generally possible with a file based backing, which prevented
 efficient streaming of request bodies. Now, `rack.input` is not required to be
-rewindable.
+rewindable. Use `Rack::RewindableInput::Middleware` to make the body rewindable.
+
+### `rack.input` is no longer rewound after being consumed
+
+Previously, `rack.input` was always rewindable, and `.rewind` was called after
+consuming form or multipart data. Call `.rewind` explicitly to consume input
+a second time.
 
 ## Response Changes