Add note about changing basePath config

[ijjk]

Closes: #19099

[ijjk]

Stats from current PR

Default Server Mode (Increase detected ⚠️)

General

	vercel/next.js canary	ijjk/next.js docs/base-path	Change
buildDuration	13.4s	13.4s	✓
nodeModulesSize	84.9 MB	84.9 MB	✓

Page Load Tests Overall increase ✓

	vercel/next.js canary	ijjk/next.js docs/base-path	Change
/ failed reqs	0	0	✓
/ total time (seconds)	2.449	2.445	0
/ avg req/sec	1020.72	1022.57	+1.85
/error-in-render failed reqs	0	0	✓
/error-in-render total time (seconds)	1.339	1.293	-0.05
/error-in-render avg req/sec	1867.01	1933.03	+66.02

Client Bundles (main, webpack, commons)

	vercel/next.js canary	ijjk/next.js docs/base-path	Change
677f882d2ed8..d4b6.js gzip	12.6 kB	12.6 kB	✓
framework.HASH.js gzip	39 kB	39 kB	✓
main-43dd9e5..8439.js gzip	6.48 kB	6.48 kB	✓
webpack-e067..f178.js gzip	751 B	751 B	✓
Overall change	58.8 kB	58.8 kB	✓

Client Bundles (main, webpack, commons) Modern

	vercel/next.js canary	ijjk/next.js docs/base-path	Change
677f882d2ed8..dule.js gzip	8.22 kB	8.22 kB	✓
framework.HA..dule.js gzip	39 kB	39 kB	✓
main-472f7d5..dule.js gzip	5.58 kB	5.58 kB	✓
webpack-07c5..dule.js gzip	751 B	751 B	✓
Overall change	53.5 kB	53.5 kB	✓

Legacy Client Bundles (polyfills)

	vercel/next.js canary	ijjk/next.js docs/base-path	Change
polyfills-4b..e242.js gzip	31 kB	31 kB	✓
Overall change	31 kB	31 kB	✓

Client Pages

	vercel/next.js canary	ijjk/next.js docs/base-path	Change
_app-3b0cf13..85f8.js gzip	1.28 kB	1.28 kB	✓
_error-6f635..c393.js gzip	3.44 kB	3.44 kB	✓
hooks-d4ffc3..9e0f.js gzip	887 B	887 B	✓
index-17468f..5d83.js gzip	227 B	227 B	✓
link-d9a15a0..130a.js gzip	1.57 kB	1.57 kB	✓
routerDirect..924c.js gzip	284 B	284 B	✓
withRouter-7..c13d.js gzip	284 B	284 B	✓
Overall change	7.98 kB	7.98 kB	✓

Client Pages Modern

	vercel/next.js canary	ijjk/next.js docs/base-path	Change
_app-75d3a82..dule.js gzip	625 B	625 B	✓
_error-f2fcc..dule.js gzip	2.29 kB	2.29 kB	✓
hooks-cbf13f..dule.js gzip	387 B	387 B	✓
index-b9a643..dule.js gzip	226 B	226 B	✓
link-973cce3..dule.js gzip	1.51 kB	1.51 kB	✓
routerDirect..dule.js gzip	284 B	284 B	✓
withRouter-f..dule.js gzip	282 B	282 B	✓
Overall change	5.6 kB	5.6 kB	✓

Client Build Manifests

	vercel/next.js canary	ijjk/next.js docs/base-path	Change
_buildManifest.js gzip	321 B	321 B	✓
_buildManife..dule.js gzip	330 B	330 B	✓
Overall change	651 B	651 B	✓

Rendered Page Sizes

	vercel/next.js canary	ijjk/next.js docs/base-path	Change
index.html gzip	970 B	970 B	✓
link.html gzip	977 B	977 B	✓
withRouter.html gzip	962 B	962 B	✓
Overall change	2.91 kB	2.91 kB	✓

---

Serverless Mode

General

	vercel/next.js canary	ijjk/next.js docs/base-path	Change
buildDuration	15.3s	15.2s	-108ms
nodeModulesSize	84.9 MB	84.9 MB	✓

Client Bundles (main, webpack, commons)

	vercel/next.js canary	ijjk/next.js docs/base-path	Change
677f882d2ed8..d4b6.js gzip	12.6 kB	12.6 kB	✓
framework.HASH.js gzip	39 kB	39 kB	✓
main-43dd9e5..8439.js gzip	6.48 kB	6.48 kB	✓
webpack-e067..f178.js gzip	751 B	751 B	✓
Overall change	58.8 kB	58.8 kB	✓

Client Bundles (main, webpack, commons) Modern

	vercel/next.js canary	ijjk/next.js docs/base-path	Change
677f882d2ed8..dule.js gzip	8.22 kB	8.22 kB	✓
framework.HA..dule.js gzip	39 kB	39 kB	✓
main-472f7d5..dule.js gzip	5.58 kB	5.58 kB	✓
webpack-07c5..dule.js gzip	751 B	751 B	✓
Overall change	53.5 kB	53.5 kB	✓

Legacy Client Bundles (polyfills)

	vercel/next.js canary	ijjk/next.js docs/base-path	Change
polyfills-4b..e242.js gzip	31 kB	31 kB	✓
Overall change	31 kB	31 kB	✓

Client Pages

	vercel/next.js canary	ijjk/next.js docs/base-path	Change
_app-3b0cf13..85f8.js gzip	1.28 kB	1.28 kB	✓
_error-6f635..c393.js gzip	3.44 kB	3.44 kB	✓
hooks-d4ffc3..9e0f.js gzip	887 B	887 B	✓
index-17468f..5d83.js gzip	227 B	227 B	✓
link-d9a15a0..130a.js gzip	1.57 kB	1.57 kB	✓
routerDirect..924c.js gzip	284 B	284 B	✓
withRouter-7..c13d.js gzip	284 B	284 B	✓
Overall change	7.98 kB	7.98 kB	✓

Client Pages Modern

	vercel/next.js canary	ijjk/next.js docs/base-path	Change
_app-75d3a82..dule.js gzip	625 B	625 B	✓
_error-f2fcc..dule.js gzip	2.29 kB	2.29 kB	✓
hooks-cbf13f..dule.js gzip	387 B	387 B	✓
index-b9a643..dule.js gzip	226 B	226 B	✓
link-973cce3..dule.js gzip	1.51 kB	1.51 kB	✓
routerDirect..dule.js gzip	284 B	284 B	✓
withRouter-f..dule.js gzip	282 B	282 B	✓
Overall change	5.6 kB	5.6 kB	✓

Client Build Manifests

	vercel/next.js canary	ijjk/next.js docs/base-path	Change
_buildManifest.js gzip	321 B	321 B	✓
_buildManife..dule.js gzip	330 B	330 B	✓
Overall change	651 B	651 B	✓

Serverless bundles

	vercel/next.js canary	ijjk/next.js docs/base-path	Change
_error.js	916 kB	916 kB	✓
404.html	4.61 kB	4.61 kB	✓
hooks.html	3.86 kB	3.86 kB	✓
index.js	916 kB	916 kB	✓
link.js	974 kB	974 kB	✓
routerDirect.js	967 kB	967 kB	✓
withRouter.js	967 kB	967 kB	✓
Overall change	4.75 MB	4.75 MB	✓

Commit: 7bf94c1