The documentation on https://doc.wikimedia.org/mediawiki-core/master/js/ (and similar for some extensions) is great, but it lacks one important information: the ResourceLoader module that contains the documented object. For example, when I'm on https://doc.wikimedia.org/mediawiki-core/master/js/#!/api/mw.widgets.TitleInputWidget it's hard to tell that I have to declare a dependency on mediawiki.widgets to use it.
Currently, the best way to get this information seems to be: Look at the name of the file containing the object (this is displayed on the documentation), go to https://phabricator.wikimedia.org/source/mediawiki/browse/master/resources/Resources.php (or the respective file for extensions), search for the filename there.
Getting the name of the module is so important that there should be an easier way to get it.