Add .d.ts comments to flow

src/error/formatError.js

@@ -22,9 +22,31 @@ export function formatError(error: GraphQLError): GraphQLFormattedError {
     : { message, locations, path };
 }
 
+/**
+ * @see https://github.com/graphql/graphql-spec/blob/master/spec/Section%207%20--%20Response.md#errors
+ */
 export type GraphQLFormattedError = {|
+  /**
+   * A short, human-readable summary of the problem that **SHOULD NOT** change
+   * from occurrence to occurrence of the problem, except for purposes of
+   * localization.
+   */
   +message: string,
+  /**
+   * If an error can be associated to a particular point in the requested
+   * GraphQL document, it should contain a list of locations.
+   */
   +locations: $ReadOnlyArray<SourceLocation> | void,
+  /**
+   * If an error can be associated to a particular field in the GraphQL result,
+   * it _must_ contain an entry with the key `path` that details the path of
+   * the response field which experienced the error. This allows clients to
+   * identify whether a null result is intentional or caused by a runtime error.
+   */
   +path: $ReadOnlyArray<string | number> | void,
+  /**
+   * Reserved for implementors to extend the protocol however they see fit,
+   * and hence there are no additional restrictions on its contents.
+   */
   +extensions?: { [key: string]: mixed, ... },
 |};