fix(v2): docusaurus start --poll 500 should work + better config load failure error

[slorber]

Motivation

Fixes #3620

docusaurus start --poll 500 should accept 500 as poll interval value

Also we should use polling for both webpack dev server and chokidar file watchers

Also improve the error message in such weird case so that user can understand we try to load the config from 500/docusaurus.config.js and that 500 is interpreted as the cli siteDir arg.

Have you read the Contributing Guidelines on pull requests?

yes

Test Plan

Local test and checking the correct args are forwarded to Webpack config

yarn start:v2
yarn start:v2 --poll
yarn start:v2 --poll 500

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[lex111]

Let's update our docs some more.
https://v2.docusaurus.io/docs/cli/#options

[slorber]

Any idea what I should add? As we already ref the webpack watchOption link, maybe I should just make it more obvious that we can pass an arg?

(btw we don't do that for port/host neither currently)

[lex111]

Any idea what I should add? As we already ref the webpack watchOption link, maybe I should just make it more obvious that we can pass an arg?

Yes, that will be enough.