There are other *.idl files in the Gecko source tree but they are not WebIDL, so you can ignore them. For Gecko, all WebIDL files are grouped in a single directory.Pre-Chromium versions of Edge used it internally, but these are unfortunately not public. Three browser engines use (modified) WebIDL as part as their toolchain: Gecko, Chromium/Blink, and WebCore/WebKit.So double check what is there with implementations (and don't hesitate to file bugs if you discover incoherence). On MDN we want to be practical and document what the Web platform really is, not what it ideally should be.
Though the canonical reference, we have to keep in mind that they may differ from the actual implementation.
In the wider world of computing there are several kinds of IDL. IDL stands for Interface Definition Language and it is designed to describe APIs. This document provides a reference to help understand WebIDL syntax. WebIDL files are a very condensed way of giving a lot, but not all, of the information about the API. When writing documentation about an API, the sources of information are many: the specifications describe what should be implemented as well as the model, and the implementations describe what has actually been put in the browsers.