The reference documentation for the Alfresco ReST API is available in what is referred to as the API Explorer application. This application is available for each version of ACS.
If you are confused about what API endpoints that are supported for a specific version of ACS, then install the associated API Explorer and check if the API endpoint is available. If you are wondering about specific API endpoint details, and you cannot find any information about it anywhere, consult the API Explorer.
You can find more information about the API Explorer on Install and Authenticate, which also has information on how to install it for your specific version of ACS.
If you want to know what the API Explorer looks like right now, then have a look at the online version at https://api-explorer.alfresco.com/api-explorer (note that this API Explorer always shows the API for the latest version of ACS).