You put your cursor on the "Range" portion of m.Range, and hit "Jump to Definition".
You can also just not import things that do crazy things with iterators. Based on the history of the community, that will actually be fairly easy. I'm yet to see anything crazy with generics get into a library that I use. I'm abundantly positive there's going to be a dozen "hey let's go do crazy things with rangefunc" libraries in the next couple of weeks, probably some will even make it to HN (with predictable comments bemoaning how complicated Go is getting even though these libraries have an expected use rate in the low dozens of people in the next few years), but the odds of them penetrating into common practice remain low. (Higher than previous attempts at such libraries, because rangefunc fixes some basic issues with them. But still low overall, I think.)
I think you could go many, many years programming Go in the next few years and not encounter any funny iterators in real code. If you just assume the iterator is doing what it looks like it should be doing and isn't doing anything funny, you're going to be 99%+ correct in the Go programming world.