remove commented code and add docs explaining the scope of the libdef

Author: LoganBarnett

decls/custom-error-instance.js

@@ -1,5 +1,9 @@
 // @flow
 
+// This is a partial libdef for custom-error-instance. It's filled out enough
+// such that it should work for the purposes of validated. However, the
+// documentation shows a potentially very complex API that might be hard to Flow
+// to express to a general purpose audience such as `flow-typed`.
 declare module 'custom-error-instance' {
   declare class CustomError extends Error {
     constructor(messageOrProps?: string | Object, properties?: Object): this;
@@ -13,11 +17,6 @@ declare module 'custom-error-instance' {
     originalMessage: string;
   }
 
-  // declare type CustomErrorFactory = (
-  //   message?: string,
-  //   config?: Object,
-  // ) => CustomError
-
   declare type CustomErrorApi = (
     name?: string,
     parent?: CustomError,