docs(reconciler): document removal of flushSync for custom renderers

[pranamya123]

This PR documents the removal of flushSync from the react-reconciler public API and clarifies the supported replacement APIs for custom renderers.

Summary

flushSync was previously exposed on the react-reconciler export and was removed in v0.31+ as part of PR #28500. After this change, accessing flushSync from a custom reconciler returns undefined with no documentation or migration guidance.

Custom renderers that followed the earlier documented pattern:

const { flushSync } = reconciler;

silently broke after upgrading.

This PR documents the intended replacement APIs for synchronous work in custom renderers:

updateContainerSync

flushSyncWork

The goal is to make this breaking change explicit and provide clear guidance for renderer authors.

How did you test this change?

• Documentation-only change.
• Verified formatting and linting locally.

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