[DOC release] Remove private wording from makeBoundHelper.

This function is required when storing helpers in the container.

Specifically, in ember-cli (or EAK/EAKR) you would need to use
`Ember.Handelbars.makeBoundHelper` in your helpers default export.

Something like the following is needed (in `app/helpers/up-case.js`):

```javascript
export default Ember.Handlebars.makeBoundHelper(function(value) {
  return value.toUpperCase();
});
```

The current "private" verbiage is misleading, and this should be a
normally supported use case.
Merge pull request #4715 from rjackson/update-makeBoundHelper-docs

[DOC release] Remove private wording from makeBoundHelper.