FWIW square brackets indicate an optional parameter and the pipe inside indicates all the parameters available; but I agree, I don't particularly care for that notation and prefer actual running SQL examples.
Railroad-esque diagrams can be weird but they say a lot in a very short space, I would highly recommend spending a little extra time working on thinking through them, they are everywhere!
Well, you should!
But, I didn't invent anything there, that's what they are named :)
Syntax diagrams (or railroad diagrams) are a way to represent a context-free grammar. They represent a graphical alternative to Backus–Naur form, EBNF, Augmented Backus–Naur form, and other text-based grammars as metalanguages...
https://en.wikipedia.org/wiki/Syntax_diagram
