It appears current F# documentation from Microsoft is missing the core library. The only documentation I could find looks quite dated and is from 2017. I spent a surprising amount of time (this being 2020 and F# being around for a while now) with google searches, trying to find the documentation for FSharpValue.MakeUnion method and could not find anything useful that appears to be from the horse's mouth.
Searching for a method the above method name in MS's documentation produces the following result (and all results on Ms docs link also leads to nothing):
Where should I be looking?