Clarify streaming narrative in RSC migration docs: distinguish HTML streaming from progressive data streaming

[justin808]

Context

PR #2803 replaced fictional async-props APIs with real stream_react_component patterns. During review, multiple reviewers flagged that the docs now conflate two distinct concepts under "streaming":

1. HTML streaming — renderToPipeableStream streams rendered HTML progressively as React renders the component tree. Works with stream_react_component + synchronous props.
2. Progressive data streaming — Slow data sources resolve independently and stream results to the browser, filling in Suspense placeholders. Requires async Server Components + Suspense boundaries (as shown in docs/pro/streaming-ssr.md).

Problem

Several sections use titles like "Progressive Streaming Architecture" and "Streaming Slow Data" but show code that loads ALL data synchronously in Rails before anything is sent. The HTML streaming is real, but it's not "progressive" in the way users expect — no data arrives incrementally.

Affected files and sections

• docs/oss/migrating/rsc-data-fetching.md — "Progressive Streaming Architecture" (line ~679), "How it works" steps (line ~126)
• docs/oss/migrating/rsc-component-patterns.md — Pattern 4 description (line ~334)
• docs/oss/migrating/rsc-context-and-state.md — Pattern 3 description (line ~127)

Additional: as_json(include:) without only: scopes

The @product.as_json(include: [:specs, :reviews]) pattern used in 3 files serializes all columns of included associations. While less sensitive than user data, it's inconsistent with the current_user.as_json(only: [...]) pattern the PR introduced. Consider either:

• Adding only: scopes to included associations
• Adding a brief comment recommending only: for production use

Suggested approach

• Accurately describe the synchronous-props pattern (all data loads before rendering starts — simpler but not progressive)
• For truly progressive streaming, show the async Server Component + Suspense pattern from docs/pro/streaming-ssr.md
• Align section titles with what the code actually demonstrates

References

• PR Fix fictional API names, missing associations, form_with, and CSRF patterns in RSC docs #2803 review threads (declined as out of scope)
• docs/pro/streaming-ssr.md for the actual progressive streaming pattern

Closes: n/a (follow-up from #2803)