Document private functions with @internal tag

[craicoverflow]

Closes #2183

Manually added an @internal JSDoc tag to exported functions that are not part of the public API.

I began by by making an ESLint rule which lives in the resources directory.

Although this helped me to identify the exported functions which are not exported in src/index.js, the rule is not yet stable enough to be included and I may look to follow this up with a separate PR, dealing exclusively with the ESLint rule.

[wtrocki]

I can confirm the same values locally. I just did not add internal flags to src/__tests__/starWarsData.js

[IvanGoncharov]

@craicoverflow Thanks a lot for PR 👍
One I didn't mention is that __tests__ & __fitures__ are not part of NPM package so all function exported from these folders are private by definition.
So can you please revert all changes under __tests__ folder?

[IvanGoncharov]

Same goes for jsutils folder:
https://github.com/graphql/graphql-js/blob/master/src/jsutils/README.md

These functions are not part of the module interface and are subject to change.

[craicoverflow]

Sure, I can change that 👍

It's maybe worth noting that in this same file, pathToArray is exported as public in src/index.js (aliased as responsePathAsArray).

graphql-js/src/index.js

Line 287 in 83d5011

responsePathAsArray,

[IvanGoncharov]

@craicoverflow Good catch 👍 I need to think about it 🤔 💭