docs: improve why usePrettierrc option is needed

[JounQin]

@BPScott I don't quite understand why we add usePrettierrc option in #83, prettierrc is used for all tools, it should always be preferred, I have no idea about in which case we should have inconsistent prettier settings, what do you think?

Maybe @eliperelman @azz @not-an-aardvark can help me to understand?

Originally posted by @JounQin in #481 (comment)

[BPScott]

#83 was before my time. It is possible to run prettier without loading the prettierrc config file via the CLI's --no-config option or through the API by calling prettier.format() without passing through the options generated by calling resolveConfig.

I suspect #83 was added to maintain parity with those options, because somebody was creating some tool that wrapped over prettier/eslint with a set of hardcoded, unconfigurable settings.

I agree that 99% of the time users should won't need this functionality, and that they should rely on settings their options in their .prettierrc file. However somebody clearly had a need for it and the maintenance overhead is negligible, so I think it is worthwhile to keep this setting.

[JounQin]

I want to understand in which case a user would want a different config between ESLint and prettier itself.

It is possible to run prettier without loading the prettierrc config file via the CLI's --no-config option or through the API by calling prettier.format() without passing through the options generated by calling resolveConfig.

This seems reasonable, so closing. But the document could be improved, so that any further maintainer won't be confused like me.

[JounQin]

close via #510