What documentation tells us that "[" can be used as a function?

Several of the answers to this question suggest that the best way to extract a subset from a list is to use something like sapply(mylist, "[", y). I found this rather disturbing, because I have never seen anything to suggest that we can use "[" as if it's a function. Where is this documented? I checked ?'[[' for my version, 3.6.3, but I can't see any reference to this functionality in that documentation.