Command go allowed in some txtar scripts and not in others, please document

[rudifa]

I was intrigued by the difference between .txtar files intended for running with testscript first.txtar and those intended for running with go test other.txtar.

The files ran by testscript support the command go with a variety of arguments, as documented in testscript -h, in addition to commands, conditions and supporting files that are interpreted by the small script engine in the testscript package; files run by go test do not support the command go.

I suggest that this difference should be mentioned and documented in testscript doc, for the benefit of newcomers.

[mvdan]

The difference is https://pkg.go.dev/github.com/rogpeppe/go-internal/gotooltest. It's true that the gotooltest package is not linked from the testscript package's documentation, so I'd happily approve a small patch doing that.

[mvdan]

Your PR above clarified this.