docs: add clarifying note about subscription callback data

[matthandlersux]

trafficstars

i recently implemented this feature in a go codebase and so i'm adding to the docs a clarifying note that would have helped save me some time.

it appears that the router currently returns a 200 even if the callback data does not match exactly what it expects, which added to my confusion. it took me some time before i realized the lack of __typename in my payload was the reason the router was not making subgraph requests to fill in extra data before passing it down to the subscriber.

WRT the markdown, i'm happy to make any edits requested (i thought the blockquote had the best visual styling for this information)

---

Checklist

Complete the checklist (and note appropriate exceptions) before the PR is marked ready-for-review.

• [x] Changes are compatible[^1]
• [x] Documentation[^2] completed
• [ ] Performance impact assessed and acceptable
• Tests added and passing[^3]
  • [ ] Unit Tests
  • [ ] Integration Tests
  • [ ] Manual Tests

Exceptions

Note any exceptions here

Notes

[^1]: It may be appropriate to bring upcoming changes to the attention of other (impacted) groups. Please endeavour to do this before seeking PR approval. The mechanism for doing this will vary considerably, so use your judgement as to how and when to do this. [^2]: Configuration is an important part of many changes. Where applicable please try to document configuration examples. [^3]: Tick whichever testing boxes are applicable. If you are adding Manual Tests, please document the manual testing (extensively) in the Exceptions.

[apollo-cla]

@matthandlersux: Thank you for submitting a pull request! Before we can merge it, you'll need to sign the Apollo Contributor License Agreement here: https://contribute.apollographql.com/

[router-perf[bot]]

CI performance tests

• [x] step - Basic stress test that steps up the number of users over time
• [ ] reload - Reload test over a long period of time at a constant rate of users
• [ ] step-with-prometheus - A copy of the step test with the Prometheus metrics exporter enabled
• [ ] events_without_dedup_callback - Stress test for events with a lot of users and deduplication DISABLED using callback mode
• [ ] events - Stress test for events with a lot of users and deduplication ENABLED
• [ ] events_without_dedup - Stress test for events with a lot of users and deduplication DISABLED
• [ ] events_big_cap_high_rate_callback - Stress test for events with a lot of users, deduplication enabled and high rate event with a big queue capacity using callback mode
• [ ] xlarge-request - Stress test with 10 MB request payload
• [x] const - Basic stress test that runs with a constant number of users
• [ ] events_big_cap_high_rate - Stress test for events with a lot of users, deduplication enabled and high rate event with a big queue capacity
• [ ] step-jemalloc-tuning - Clone of the basic stress test for jemalloc tuning
• [ ] xxlarge-request - Stress test with 100 MB request payload
• [ ] demand-control-uninstrumented - A copy of the step test, but with demand control monitoring enabled
• [ ] no-graphos - Basic stress test, no GraphOS.
• [ ] events_callback - Stress test for events with a lot of users and deduplication ENABLED in callback mode
• [ ] large-request - Stress test with a 1 MB request payload
• [ ] demand-control-instrumented - A copy of the step test, but with demand control monitoring and metrics enabled