You can find the documentation for the ArcGIS API for JavaScript by visiting https://developers.arcgis.com/javascript/jsapi/. There, you'll find information on the latest version of the API, a list of changes since the previous version, and related documents to help you create apps. The organization is logical, and information is relatively easy to find with their layout.
The documentation for each element in the ArcGIS API is laid out in an organized fashion. Links to the API components are on the left-hand side. The documentation for each element defaults to the AMD style, but provides links to the older legacy development style. With the AMD style, the top of the page shows how to load the module into your code. Following that is a description of the module, a link to samples that use the module, and a class hierarchy diagram showing the modules the current module inherits.