Add note explaining empty query during prerendering

[ijjk]

A few Spectrum threads have been confused about this behavior, this adds some explanation to help clear up the query behavior during prerendering.

Related threads:
https://spectrum.chat/next-js/general/withrouter-router-query-is-empty-serverside~bdb81c8e-56dc-4125-985a-e8f0609c5d17
https://spectrum.chat/next-js/general/how-do-you-get-url-params-serverside~03e18a52-367f-4fe5-b748-69b9b2354f9a
https://spectrum.chat/next-js/general/nextjs-9-props-router-query-empty-without-getinitialprops~ea7582aa-7f7e-4741-acd3-68288df518e2

[ijjk]

Stats from current PR

Default Server Mode

General

	zeit/next.js canary	ijjk/next.js document-prerender-query	Change
buildDuration	23.5s	23.3s	-256ms
nodeModulesSize	41.5 MB	41.5 MB	✓

Client Bundles (main, webpack, commons)

	zeit/next.js canary	ijjk/next.js document-prerender-query	Change
main-HASH.js	13.8 kB	13.8 kB	✓
main-HASH.js gzip	4.87 kB	4.87 kB	✓
webpack-HASH.js	1.53 kB	1.53 kB	✓
webpack-HASH.js gzip	746 B	746 B	✓
commons.HASH.js	194 kB	194 kB	✓
commons.HASH.js gzip	63.1 kB	63.1 kB	✓
Overall change			✓

Client Bundles (main, webpack, commons) Modern

	zeit/next.js canary	ijjk/next.js document-prerender-query	Change
main-HASH.module.js	11.4 kB	11.4 kB	✓
main-HASH.module.js gzip	4.31 kB	4.31 kB	✓
webpack-HASH.module.js	1.53 kB	1.53 kB	✓
webpack-HASH.module.js gzip	746 B	746 B	✓
commons.HASH.module.js	175 kB	175 kB	✓
commons.HASH.module.js gzip	57.1 kB	57.1 kB	✓
Overall change			✓

Client Pages

	zeit/next.js canary	ijjk/next.js document-prerender-query	Change
_app.js	1.9 kB	1.9 kB	✓
_app.js gzip	905 B	905 B	✓
_error.js	8.02 kB	8.02 kB	✓
_error.js gzip	3.12 kB	3.12 kB	✓
index.js	343 B	343 B	✓
index.js gzip	237 B	237 B	✓
link.js	4.07 kB	4.07 kB	✓
link.js gzip	1.79 kB	1.79 kB	✓
routerDirect.js	423 B	423 B	✓
routerDirect.js gzip	290 B	290 B	✓
withRouter.js	435 B	435 B	✓
withRouter.js gzip	287 B	287 B	✓
Overall change			✓

Client Pages Modern

	zeit/next.js canary	ijjk/next.js document-prerender-query	Change
_app.module.js	1.79 kB	1.79 kB	✓
_app.module.js gzip	865 B	865 B	✓
_error.module.js	5.82 kB	5.82 kB	✓
_error.module.js gzip	2.33 kB	2.33 kB	✓
index.module.js	319 B	319 B	✓
index.module.js gzip	238 B	238 B	✓
link.module.js	3.78 kB	3.78 kB	✓
link.module.js gzip	1.7 kB	1.7 kB	✓
routerDirect.module.js	411 B	411 B	✓
routerDirect.module.js gzip	291 B	291 B	✓
withRouter.module.js	423 B	423 B	✓
withRouter.module.js gzip	288 B	288 B	✓
Overall change			✓

Fetched pages

	zeit/next.js canary	ijjk/next.js document-prerender-query	Change
link	2.81 kB	2.81 kB	✓
link gzip	844 B	843 B	-1 B
index	2.76 kB	2.76 kB	✓
index gzip	836 B	833 B	-3 B
withRouter	2.82 kB	2.82 kB	✓
withRouter gzip	831 B	828 B	-3 B
Overall change			✓

---

Serverless Mode

General

	zeit/next.js canary	ijjk/next.js document-prerender-query	Change
buildDuration	25.9s	24.2s	-1.7s
nodeModulesSize	41.5 MB	41.5 MB	✓

Client Bundles (main, webpack, commons)

	zeit/next.js canary	ijjk/next.js document-prerender-query	Change
main-HASH.js	13.8 kB	13.8 kB	✓
main-HASH.js gzip	4.87 kB	4.87 kB	✓
webpack-HASH.js	1.53 kB	1.53 kB	✓
webpack-HASH.js gzip	746 B	746 B	✓
commons.HASH.js	194 kB	194 kB	✓
commons.HASH.js gzip	63.1 kB	63.1 kB	✓
Overall change			✓

Client Bundles (main, webpack, commons) Modern

	zeit/next.js canary	ijjk/next.js document-prerender-query	Change
main-HASH.module.js	11.4 kB	11.4 kB	✓
main-HASH.module.js gzip	4.31 kB	4.31 kB	✓
webpack-HASH.module.js	1.53 kB	1.53 kB	✓
webpack-HASH.module.js gzip	746 B	746 B	✓
commons.HASH.module.js	175 kB	175 kB	✓
commons.HASH.module.js gzip	57.1 kB	57.1 kB	✓
Overall change			✓

Client Pages

	zeit/next.js canary	ijjk/next.js document-prerender-query	Change
_app.js	1.9 kB	1.9 kB	✓
_app.js gzip	905 B	905 B	✓
_error.js	8.02 kB	8.02 kB	✓
_error.js gzip	3.12 kB	3.12 kB	✓
index.js	343 B	343 B	✓
index.js gzip	237 B	237 B	✓
link.js	4.07 kB	4.07 kB	✓
link.js gzip	1.79 kB	1.79 kB	✓
routerDirect.js	423 B	423 B	✓
routerDirect.js gzip	290 B	290 B	✓
withRouter.js	435 B	435 B	✓
withRouter.js gzip	287 B	287 B	✓
Overall change			✓

Client Pages Modern

	zeit/next.js canary	ijjk/next.js document-prerender-query	Change
_app.module.js	1.79 kB	1.79 kB	✓
_app.module.js gzip	865 B	865 B	✓
_error.module.js	5.82 kB	5.82 kB	✓
_error.module.js gzip	2.33 kB	2.33 kB	✓
index.module.js	319 B	319 B	✓
index.module.js gzip	238 B	238 B	✓
link.module.js	3.78 kB	3.78 kB	✓
link.module.js gzip	1.7 kB	1.7 kB	✓
routerDirect.module.js	411 B	411 B	✓
routerDirect.module.js gzip	291 B	291 B	✓
withRouter.module.js	423 B	423 B	✓
withRouter.module.js gzip	288 B	288 B	✓
Overall change			✓

Serverless bundles

	zeit/next.js canary	ijjk/next.js document-prerender-query	Change
_error.js	247 kB	247 kB	✓
_error.js gzip	66.2 kB	66.2 kB	-1 B
index.js	247 kB	247 kB	✓
index.js gzip	66.5 kB	66.5 kB	-1 B
link.js	255 kB	255 kB	✓
link.js gzip	68.7 kB	68.7 kB	-1 B
routerDirect.js	248 kB	248 kB	✓
routerDirect.js gzip	66.4 kB	66.4 kB	-1 B
withRouter.js	248 kB	248 kB	✓
withRouter.js gzip	66.5 kB	66.5 kB	-2 B
Overall change			✓