[Automated][tcgc][skip chg] Update TCGC override response documentation

## Summary

- clarify that plain `@override` return types are ignored and do not trigger response compatibility diagnostics
- document that `replaceResponseWithVoid` and `replaceResponseWithBytes` are the supported response replacement mechanisms
- regenerate decorator reference docs and update the documentation updater knowledge and metadata

## Validation

- TCGC dependency build
- 28 targeted tests
- reference documentation regeneration
- `pnpm format`
- `pnpm lint`
- cspell and `git diff --check`

> Generated by [Documentation Update Agent](https://github.com/Azure/typespec-azure/actions/runs/34826696485) · gpt56 · 289.7 AIC · ⌖ 32.9 AIC · ⊞ 19.1K · [◷](https://github.com/search?q=repo%3AAzure%2Ftypespec-azure+%22gh-aw-workflow-id%3A+doc-update%22&type=pullrequests)

---

> [!NOTE]
> This was originally intended as a pull request, but GitHub Actions is not permitted to create or approve pull requests in this repository.
> The changes have been pushed to branch `docs/tcgc-incremental-20260914-3e1a8a4f4a55a898`.
>
> **[Click here to create the pull request](https://github.com/Azure/typespec-azure/compare/main...docs/tcgc-incremental-20260914-3e1a8a4f4a55a898?expand=1&title=%5BAutomated%5D%5Btcgc%5D%5Bskip%20chg%5D%20Update%20TCGC%20override%20response%20documentation)**

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Show patch preview (132 of 132 lines)

```diff
From 7a739d678a31f5af4611fbb9b46b79b1be9b5eea Mon Sep 17 00:00:00 2001
X-GH-AW-Base-Commit: 5f4088cb98034d388da0ca32db6c2172103c3abe
From: "github-actions[bot]"
Date: Mon, 14 Sep 2026 10:16:29 +0000
Subject: [PATCH] Update TCGC override response documentation

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
---
eng/scripts/doc-updater/knowledge/tcgc.md | 3 ++-
eng/scripts/doc-updater/knowledge/tcgc.meta.json | 4 ++--
packages/typespec-client-generator-core/lib/decorators.tsp | 6 ++++--
.../docs/docs/howtos/Generate client libraries/04method.mdx | 6 +++---
.../libraries/typespec-client-generator-core/guideline.md | 2 +-
.../typespec-client-generator-core/reference/decorators.md | 6 ++++--
6 files changed, 16 insertions(+), 11 deletions(-)

diff --git a/eng/scripts/doc-updater/knowledge/tcgc.md b/eng/scripts/doc-updater/knowledge/tcgc.md
index de685f2f1..22cf5e2ea 100644
--- a/eng/scripts/doc-updater/knowledge/tcgc.md
+++ b/eng/scripts/doc-updater/knowledge/tcgc.md
@@ -19,7 +19,7 @@
5. `@operationGroup(target, scope?)` — DEPRECATED, use @client
6. `@usage(target, value, scope?)` — mark model/enum/union/namespace usage (input/output/json/xml); on namespace, propagates recursively to all contained types
7. `@access(target, value, scope?)` — public/internal visibility
-8. `@override(target, override, scope?)` — customize method signatures
+8. `@override(target, override, scope?)` — customize method parameters; a plain override's declared return type is ignored, while response replacement requires `replaceResponseWithVoid` or `replaceResponseWithBytes`
9. `@useSystemTextJsonConverter(target, scope?)` — C# backward compat only
10. `@clientInitialization(target, options, scope?)` — customize client init; options has parameters model and initializedBy flags
11. `@paramAlias(target, alias, scope?)` — alias client init parameter names
@@ -118,6 +118,7 @@ namespace
... (truncated)
```

Start with packages/typespec-client-generator-core/lib/decorators.tsp and the related reference, guideline, how-to, and doc-updater knowledge files listed in the patch preview. Run the 28 targeted tests, reference documentation regeneration, pnpm format, pnpm lint, cspell, and git diff --check; done means the TCGC override response behavior is consistently documented across those files.

Written by the indexing model from the issue text.