text: Title: docs: little updates
  Body: Add to [this section](https://faststream.airt.ai/latest/getting-started/subscription/annotation/#json-basic-serialization)
  * [ ] partial body consuming example (https://github.com/airtai/faststream/pull/890#issuecomment-1835856313)
  * [ ] detail serialization rule notice (#1152)
  * [ ] edit [FastAPI broker section](https://faststream.airt.ai/latest/getting-started/integrations/fastapi/#accessing-the-broker-object) to use context (also test it with multiple routers)