Add documentation for the executeSync method

[brybrophy]

My company recently discovered the undocumented executeSync method that is exposed in graphql-js. We would like to use it to synchronously generate schema based mock data for our tests, however, we are wary of using an undocumented API.

Is this method intended for internal use only, or is it safe to assume that no breaking changes will be introduced? If this method is safe for external users to consume, it would be great to add it to the documentation.

[brybrophy]

Actually, it seems that all of these exports from execution/index are undocumented. So I would like to extend my question to ask if these are all intended for external consumption.

graphql-js/src/index.js

Lines 277 to 285 in 735975d

	// Execute GraphQL queries.
	export {
	execute,
	executeSync,
	defaultFieldResolver,
	defaultTypeResolver,
	responsePathAsArray,
	getDirectiveValues,
	} from './execution/index';