Eh, it's a matter of taste. Lots of people use <> to denote params,
but I prefer to assign special meaning to as few characters as possible.
The [] already suggest that it's a parameter (and that it's optional), and
the plural suggests that you can give more than one of them.

E.g. the go tool does this in `go help build|head -1`.
And so does `hk help|head -1` :)

Basically my philosophy is that this string is meant for humans to read.
It doesn't have to be pedantically unambiguous as long as it's understandable.
And cluttering the description with extra symbols can hinder understandability
just by adding visual noise.