If you just call go doc gl there would be no way for it to know which gl package you meant so it just selects the first found "package" and that so happens to be github.com/go-gl/gl which contains no source files.
So the proper way to use go doc would be to use the full import path go doc github.com/go-gl/gl/v3.3-core/gl
understood, and this isn't hugely important, but i do think go's tooling is one of its bright points, so..
"github.com/go-gl/gl" isn't even a package, there is only one package 'gl' installed, doesn't seem that hard for it to just keep going until it found a package, or, seems like a tree search leaf first would work, mostly.
also; if i already had a package 'gl' which was reporting documentation with 'gl', installing the above package, as a side-effect, breaks that, that's surprising to me, i would have hoped package installation was completely isolated. in some sort of shared development environment i might not be aware of other installs until it breaks.