mIRC's help file is actually one of the best help files you will find that comes packaged with a program used to script/program something. Probably not the best, but it's right up there at the top. Most programming software will give you a small number of examples and very short description or simply the syntax of individual commands. They usually don't provide a list of what works and doesn't work or how to do specific things. mIRC's help file often has examples and usually has pretty decent descriptions rather than just syntax. Most other programming software expects you to learn to program somewhere else and the help file is just a minor reference that really isn't all that helpful except for checking syntax. mIRC provides you with enough in the help file to write many commonly used scripts without ever resorting to any other information (at least, if you know anything about programming/scripting beforehand). And for what it doesn't provide, then you are expected to look elsewhere, just like any other language. A help file that is a few thousand pages because of being filled with information that isn't really necessary becomes useless.
If anyone remembers the old DOS manuals, they were very thick manuals that were filled with a lot of useless information that made finding useful information difficult. Since then, manuals and help files have been significantly trimmed down and cleaned up so they are useful. Yes, it means there may be something missing that you wish was there, but it means that everything else is easy to find. And it's easy enough to find information that isn't included in the help file.
This forum is a very good resource for any questions. You may have trouble finding something specific using Search depending on how you try to search for it, but if you ask a question, you will get an answer. It may require waiting a little for the answer, but you'll get it. If you're in a hurry, there are a variety of scripting channels where you can quickly receive help.
When I started scripting in mIRC, I had already had many years of programming experience, so I did know enough to understand much the language before I even started. It was mostly just syntax that I needed to learn and any of the more advanced or specific things in mIRC. I learned by taking a script (a trivia bot script [I think TrivBot]) and trying to improve on it. I mostly used the help file and when I needed more information, I used #scripting on Dalnet. That's all I needed to improve that script and learn how to script in mSL. After I learned the basics that way, I moved on to using a combination of the help file and this forum. Mostly I just use the help file. It's rare if I need to ask in the forum. It also helps that I read all posts on this forum by checking the site daily. Doing so lets me learn from other people's scripts and also try to solve scripting problems for people. That alone has been a great help in learning the language and especially the more advanced things you can do.
What it comes down to is that you need to use other sources as well as the help file. The help file is not the only source for information and it's not meant to be the only source. It's meant as *one* resource only. A very useful one, but still only one of many. As I mentioned before and others have also said, *yes*, the help file can be improved in certain ways. But it doesn't need to provide a list of identifiers that are $null for every event or to tell you if an event is used for a channel or a network or something else.
As mentioned, the events are clear in what they relate to. If it has a channel (location) field, then it is for use in a channel (or PM or whatever). If it does not, then it isn't. on QUIT does not have a location field, so that should give you a good indication that it is not related to the channel at all and so $chan would be $null. The same goes for on EXIT, on CONNECT, on DISCONNECT, on LOAD, on START, etc. None of those have a location field, so $chan will always be $null in those events and any similar events. Perhaps that isn't spelled out for people in large bold text in every event, but it can easily be inferred from looking at the various events and examples in the help file.