[Docs] Remove false caveat in custom document docs

[lfades]

The statement is not true, we provide mocks for req and res.

[ijjk]

Stats from current PR

Default Server Mode (Decrease detected ✓)

General

	vercel/next.js canary	lfades/next.js docs/remove-custom-document-caveat	Change
buildDuration	12.8s	12.4s	-334ms
nodeModulesSize	66.4 MB	66.4 MB	✓

Page Load Tests Overall decrease ⚠️

	vercel/next.js canary	lfades/next.js docs/remove-custom-document-caveat	Change
/ failed reqs	0	0	✓
/ total time (seconds)	1.952	2.024	⚠️ +0.07
/ avg req/sec	1280.76	1235.47	⚠️ -45.29
/error-in-render failed reqs	0	0	✓
/error-in-render total time (seconds)	1.236	1.226	-0.01
/error-in-render avg req/sec	2022.97	2039.77	+16.8

Client Bundles (main, webpack, commons)

	vercel/next.js canary	lfades/next.js docs/remove-custom-document-caveat	Change
677f882d2ed8..7a20.js gzip	9.85 kB	9.85 kB	✓
framework.HASH.js gzip	39.1 kB	39.1 kB	✓
main-d8a3f1c..06ea.js gzip	6.72 kB	6.72 kB	✓
polyfills-05..1236.js gzip	30.8 kB	30.8 kB	✓
webpack-488d..c0e7.js gzip	751 B	751 B	✓
Overall change	87.3 kB	87.3 kB	✓

Client Bundles (main, webpack, commons) Modern

	vercel/next.js canary	lfades/next.js docs/remove-custom-document-caveat	Change
677f882d2ed8..dule.js gzip	6.12 kB	6.12 kB	✓
framework.HA..dule.js gzip	39.1 kB	39.1 kB	✓
main-e63c4e5..dule.js gzip	5.79 kB	5.79 kB	✓
polyfills-05..dule.js gzip	30.8 kB	30.8 kB	✓
webpack-4f62..dule.js gzip	751 B	751 B	✓
Overall change	82.6 kB	82.6 kB	✓

Client Pages

	vercel/next.js canary	lfades/next.js docs/remove-custom-document-caveat	Change
_app-8f5f611..1f7b.js gzip	1.28 kB	1.28 kB	✓
_error-8e69b..8695.js gzip	3.41 kB	3.41 kB	✓
hooks-f7f3d0..7465.js gzip	887 B	887 B	✓
index-08fb3f..c0e9.js gzip	227 B	227 B	✓
link-ddd176e..5566.js gzip	1.29 kB	1.29 kB	✓
routerDirect..8aa1.js gzip	284 B	284 B	✓
withRouter-f..e777.js gzip	284 B	284 B	✓
Overall change	7.66 kB	7.66 kB	✓

Client Pages Modern

	vercel/next.js canary	lfades/next.js docs/remove-custom-document-caveat	Change
_app-669dbe5..dule.js gzip	626 B	626 B	✓
_error-663e6..dule.js gzip	2.24 kB	2.24 kB	✓
hooks-805c40..dule.js gzip	387 B	387 B	✓
index-6ba5a4..dule.js gzip	226 B	226 B	✓
link-69bc264..dule.js gzip	1.25 kB	1.25 kB	✓
routerDirect..dule.js gzip	284 B	284 B	✓
withRouter-d..dule.js gzip	282 B	282 B	✓
Overall change	5.29 kB	5.29 kB	✓

Client Build Manifests

	vercel/next.js canary	lfades/next.js docs/remove-custom-document-caveat	Change
_buildManifest.js gzip	275 B	275 B	✓
_buildManife..dule.js gzip	281 B	281 B	✓
Overall change	556 B	556 B	✓

Rendered Page Sizes

	vercel/next.js canary	lfades/next.js docs/remove-custom-document-caveat	Change
index.html gzip	946 B	946 B	✓
link.html gzip	953 B	953 B	✓
withRouter.html gzip	938 B	938 B	✓
Overall change	2.84 kB	2.84 kB	✓

---

Serverless Mode

General

	vercel/next.js canary	lfades/next.js docs/remove-custom-document-caveat	Change
buildDuration	14.1s	13.8s	-274ms
nodeModulesSize	66.4 MB	66.4 MB	✓

Client Bundles (main, webpack, commons)

	vercel/next.js canary	lfades/next.js docs/remove-custom-document-caveat	Change
677f882d2ed8..7a20.js gzip	9.85 kB	9.85 kB	✓
framework.HASH.js gzip	39.1 kB	39.1 kB	✓
main-d8a3f1c..06ea.js gzip	6.72 kB	6.72 kB	✓
polyfills-05..1236.js gzip	30.8 kB	30.8 kB	✓
webpack-488d..c0e7.js gzip	751 B	751 B	✓
Overall change	87.3 kB	87.3 kB	✓

Client Bundles (main, webpack, commons) Modern

	vercel/next.js canary	lfades/next.js docs/remove-custom-document-caveat	Change
677f882d2ed8..dule.js gzip	6.12 kB	6.12 kB	✓
framework.HA..dule.js gzip	39.1 kB	39.1 kB	✓
main-e63c4e5..dule.js gzip	5.79 kB	5.79 kB	✓
polyfills-05..dule.js gzip	30.8 kB	30.8 kB	✓
webpack-4f62..dule.js gzip	751 B	751 B	✓
Overall change	82.6 kB	82.6 kB	✓

Client Pages

	vercel/next.js canary	lfades/next.js docs/remove-custom-document-caveat	Change
_app-8f5f611..1f7b.js gzip	1.28 kB	1.28 kB	✓
_error-8e69b..8695.js gzip	3.41 kB	3.41 kB	✓
hooks-f7f3d0..7465.js gzip	887 B	887 B	✓
index-08fb3f..c0e9.js gzip	227 B	227 B	✓
link-ddd176e..5566.js gzip	1.29 kB	1.29 kB	✓
routerDirect..8aa1.js gzip	284 B	284 B	✓
withRouter-f..e777.js gzip	284 B	284 B	✓
Overall change	7.66 kB	7.66 kB	✓

Client Pages Modern

	vercel/next.js canary	lfades/next.js docs/remove-custom-document-caveat	Change
_app-669dbe5..dule.js gzip	626 B	626 B	✓
_error-663e6..dule.js gzip	2.24 kB	2.24 kB	✓
hooks-805c40..dule.js gzip	387 B	387 B	✓
index-6ba5a4..dule.js gzip	226 B	226 B	✓
link-69bc264..dule.js gzip	1.25 kB	1.25 kB	✓
routerDirect..dule.js gzip	284 B	284 B	✓
withRouter-d..dule.js gzip	282 B	282 B	✓
Overall change	5.29 kB	5.29 kB	✓

Client Build Manifests

	vercel/next.js canary	lfades/next.js docs/remove-custom-document-caveat	Change
_buildManifest.js gzip	275 B	275 B	✓
_buildManife..dule.js gzip	281 B	281 B	✓
Overall change	556 B	556 B	✓

Serverless bundles

	vercel/next.js canary	lfades/next.js docs/remove-custom-document-caveat	Change
_error.js	881 kB	881 kB	✓
404.html	4.18 kB	4.18 kB	✓
hooks.html	3.82 kB	3.82 kB	✓
index.js	881 kB	881 kB	✓
link.js	923 kB	923 kB	✓
routerDirect.js	917 kB	917 kB	✓
withRouter.js	917 kB	917 kB	✓
Overall change	4.53 MB	4.53 MB	✓

Commit: 0eac72e