[Azure Functions/DTS] Document orchestration completion-request size limits #290

Parent: #269 — ask 2.

Problem

The published Durable Task Scheduler (DTS) limits describe per-activity input/output limits, but the reporter could not find documentation for the aggregate orchestration completion-request limit or that newly scheduled activity inputs count toward it.

The reported workload aggregated 121 activity results and scheduled another activity with that list as input. DTS rejected an 11.05 MB orchestration work-item completion request against a reported 3.90 MB limit, even though the individual activity results were only 2–48 KB.

Requested outcome

Document the orchestration completion-request limit separately from per-activity payload limits, including the contribution of scheduled-activity inputs to the request's total size.

Acceptance criteria

• Confirm the authoritative limit, its units, and any relevant backend/version qualifications rather than treating the rounded 3.90 MB error text as the specification.
• Explain which data counts toward the aggregate request, explicitly including newly scheduled activity inputs.
• Explain why individually small payloads can still exceed the limit during fan-in followed by additional scheduling.
• Describe mitigation through smaller/batched payloads or configured large-payload externalization, and link the Python Functions configuration guidance where applicable.
• Update the relevant public DTS/Functions documentation, coordinating with its owning repository as needed.

See #269 for the full reproduction context. Python Functions large-payload configuration was added by #270; simpler storage configuration is separately tracked in #271.