Mention header overriding behavior

[ijjk]

Closes: #16088

[ijjk]

Stats from current PR

Default Server Mode (Increase detected ⚠️)

General Overall decrease ✓

	vercel/next.js canary	ijjk/next.js docs/headers-overriding	Change
buildDuration	11.6s	11.7s	⚠️ +93ms
nodeModulesSize	57.3 MB	57.3 MB	-9.84 kB

Page Load Tests Overall increase ✓

	vercel/next.js canary	ijjk/next.js docs/headers-overriding	Change
/ failed reqs	0	0	✓
/ total time (seconds)	2.158	2.081	-0.08
/ avg req/sec	1158.42	1201.17	+42.75
/error-in-render failed reqs	0	0	✓
/error-in-render total time (seconds)	1.152	1.109	-0.04
/error-in-render avg req/sec	2169.76	2253.92	+84.16

Client Bundles (main, webpack, commons)

	vercel/next.js canary	ijjk/next.js docs/headers-overriding	Change
677f882d2ed8..6aa6.js gzip	10 kB	10 kB	✓
framework.HASH.js gzip	39.1 kB	39.1 kB	✓
main-9a7a5f4..661e.js gzip	6.71 kB	6.71 kB	✓
webpack-ccf5..276a.js gzip	751 B	751 B	✓
Overall change	56.6 kB	56.6 kB	✓

Client Bundles (main, webpack, commons) Modern

	vercel/next.js canary	ijjk/next.js docs/headers-overriding	Change
677f882d2ed8..dule.js gzip	5.92 kB	5.92 kB	✓
framework.HA..dule.js gzip	39.1 kB	39.1 kB	✓
main-d0ac695..dule.js gzip	5.79 kB	5.79 kB	✓
webpack-10c7..dule.js gzip	751 B	751 B	✓
Overall change	51.6 kB	51.6 kB	✓

Legacy Client Bundles (polyfills)

	vercel/next.js canary	ijjk/next.js docs/headers-overriding	Change
polyfills-75..1629.js gzip	31 kB	31 kB	✓
Overall change	31 kB	31 kB	✓

Client Pages

	vercel/next.js canary	ijjk/next.js docs/headers-overriding	Change
_app-874bd8a..0103.js gzip	1.28 kB	1.28 kB	✓
_error-fa39c..ec40.js gzip	3.45 kB	3.45 kB	✓
hooks-585f07..95a3.js gzip	887 B	887 B	✓
index-c7b63f..fc02.js gzip	227 B	227 B	✓
link-4c2bd9b..eadd.js gzip	1.29 kB	1.29 kB	✓
routerDirect..ebc7.js gzip	284 B	284 B	✓
withRouter-2..db68.js gzip	284 B	284 B	✓
Overall change	7.71 kB	7.71 kB	✓

Client Pages Modern

	vercel/next.js canary	ijjk/next.js docs/headers-overriding	Change
_app-97e743e..dule.js gzip	626 B	626 B	✓
_error-b4004..dule.js gzip	2.3 kB	2.3 kB	✓
hooks-696209..dule.js gzip	387 B	387 B	✓
index-a4dd74..dule.js gzip	226 B	226 B	✓
link-236a801..dule.js gzip	1.26 kB	1.26 kB	✓
routerDirect..dule.js gzip	284 B	284 B	✓
withRouter-1..dule.js gzip	282 B	282 B	✓
Overall change	5.37 kB	5.37 kB	✓

Client Build Manifests

	vercel/next.js canary	ijjk/next.js docs/headers-overriding	Change
_buildManifest.js gzip	273 B	273 B	✓
_buildManife..dule.js gzip	279 B	279 B	✓
Overall change	552 B	552 B	✓

Rendered Page Sizes

	vercel/next.js canary	ijjk/next.js docs/headers-overriding	Change
index.html gzip	945 B	945 B	✓
link.html gzip	954 B	954 B	✓
withRouter.html gzip	939 B	939 B	✓
Overall change	2.84 kB	2.84 kB	✓

---

Serverless Mode

General

	vercel/next.js canary	ijjk/next.js docs/headers-overriding	Change
buildDuration	13.1s	12.1s	-1.1s
nodeModulesSize	57.3 MB	57.3 MB	✓

Client Bundles (main, webpack, commons)

	vercel/next.js canary	ijjk/next.js docs/headers-overriding	Change
677f882d2ed8..6aa6.js gzip	10 kB	10 kB	✓
framework.HASH.js gzip	39.1 kB	39.1 kB	✓
main-9a7a5f4..661e.js gzip	6.71 kB	6.71 kB	✓
webpack-ccf5..276a.js gzip	751 B	751 B	✓
Overall change	56.6 kB	56.6 kB	✓

Client Bundles (main, webpack, commons) Modern

	vercel/next.js canary	ijjk/next.js docs/headers-overriding	Change
677f882d2ed8..dule.js gzip	5.92 kB	5.92 kB	✓
framework.HA..dule.js gzip	39.1 kB	39.1 kB	✓
main-d0ac695..dule.js gzip	5.79 kB	5.79 kB	✓
webpack-10c7..dule.js gzip	751 B	751 B	✓
Overall change	51.6 kB	51.6 kB	✓

Legacy Client Bundles (polyfills)

	vercel/next.js canary	ijjk/next.js docs/headers-overriding	Change
polyfills-75..1629.js gzip	31 kB	31 kB	✓
Overall change	31 kB	31 kB	✓

Client Pages

	vercel/next.js canary	ijjk/next.js docs/headers-overriding	Change
_app-874bd8a..0103.js gzip	1.28 kB	1.28 kB	✓
_error-fa39c..ec40.js gzip	3.45 kB	3.45 kB	✓
hooks-585f07..95a3.js gzip	887 B	887 B	✓
index-c7b63f..fc02.js gzip	227 B	227 B	✓
link-4c2bd9b..eadd.js gzip	1.29 kB	1.29 kB	✓
routerDirect..ebc7.js gzip	284 B	284 B	✓
withRouter-2..db68.js gzip	284 B	284 B	✓
Overall change	7.71 kB	7.71 kB	✓

Client Pages Modern

	vercel/next.js canary	ijjk/next.js docs/headers-overriding	Change
_app-97e743e..dule.js gzip	626 B	626 B	✓
_error-b4004..dule.js gzip	2.3 kB	2.3 kB	✓
hooks-696209..dule.js gzip	387 B	387 B	✓
index-a4dd74..dule.js gzip	226 B	226 B	✓
link-236a801..dule.js gzip	1.26 kB	1.26 kB	✓
routerDirect..dule.js gzip	284 B	284 B	✓
withRouter-1..dule.js gzip	282 B	282 B	✓
Overall change	5.37 kB	5.37 kB	✓

Client Build Manifests

	vercel/next.js canary	ijjk/next.js docs/headers-overriding	Change
_buildManifest.js gzip	273 B	273 B	✓
_buildManife..dule.js gzip	279 B	279 B	✓
Overall change	552 B	552 B	✓

Serverless bundles

	vercel/next.js canary	ijjk/next.js docs/headers-overriding	Change
_error.js	1.03 MB	1.03 MB	✓
404.html	4.18 kB	4.18 kB	✓
hooks.html	3.82 kB	3.82 kB	✓
index.js	1.03 MB	1.03 MB	✓
link.js	1.07 MB	1.07 MB	✓
routerDirect.js	1.06 MB	1.06 MB	✓
withRouter.js	1.06 MB	1.06 MB	✓
Overall change	5.26 MB	5.26 MB	✓

Commit: 29e47f1