docs(migration): return after sending 400

This is a change to ensure that users that migrate to v4 do not run
`next()` when the response was already sent.

Author: slagiewka

docs/source/migration.mdx

@@ -1121,6 +1121,7 @@ app.use((req, res, next) => {
     } catch (e) {
       // https://github.com/graphql/graphql-over-http/blob/main/spec/GraphQLOverHTTP.md#json-parsing-failure
       res.status(400).send(e instanceof Error ? e.message : e);
+      return;
     }
   }
   next();

packages/server/src/__tests__/express4/expressSpecific.test.ts

@@ -163,6 +163,7 @@ it('supporting doubly-encoded variables example from migration guide', async ()
       } catch (e) {
         // https://github.com/graphql/graphql-over-http/blob/main/spec/GraphQLOverHTTP.md#json-parsing-failure
         res.status(400).send(e instanceof Error ? e.message : e);
+        return;
       }
     }
     next();