Clarify doc regarding extension option

[pvdlg]

Ref #428 (comment)

[pvdlg]

So that's a consequence of #425 that I didn't really forsee.
If this limitation is an issue I can change the code to:

• First glob for XO config files
• For each config file get the extension option
• Glob the files to lint, taking in account the extension config
• Then call mergeWithFileConfigs

That would allow us to retain the ability to use extensions inside a config file rather than limiting that to CLI arg only. The trade-off is that we would glob 1 + n times (where n is the number of XO config found) instead off once. In the most common case (1 XO config) we would glob 2 times instead of once.

[sindresorhus]

That would allow us to retain the ability to use extensions inside a config file rather than limiting that to CLI arg only.

Yeah, I think this would be the better and expected behavior. ESLint 7 will also allow specifying extensions in config files: https://eslint.org/blog/2020/02/whats-coming-in-eslint-7.0.0#file-extensions-in-config-files (Maybe look at how they do it?)