doc/go1.13: highlight need to set GOPRIVATE when using private repos #33796
Description
What did you do?
I update to go1.13beta1 and used go mod tidy to download private dependencies and it din't work as I had not set GOPRIVATE.
I know it's documented under Tools > Modules > Environment variables. However besides myself other colleges had similar issues when using go 1.13 and I believe others would face similar problems when updating. Therefore I'd like to suggest to state in the Introduction to Go 1.13 section the need/introduction of GOPRIVATE.