_content/doc: mention GOPRIVATE in FAQ

Fixes golang/go#71797

Change-Id: I4f492a143837ebe00023d8f5bbcf59b44295c82d
Reviewed-on: https://go-review.googlesource.com/c/website/+/650336
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Michael Knyszek <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>
Auto-Submit: Ian Lance Taylor <[email protected]>

Author: seankhliao

_content/doc/faq.md

@@ -1064,6 +1064,9 @@ add these lines to your `~/.gitconfig`:
 	insteadOf = https://github.com/
 ```
 
+When working with private modules, but using a public module proxy for dependencies, you may need to set `GOPRIVATE`.
+See [private modules](/ref/mod#private-modules) for details and additional settings.
+
 ### How should I manage package versions using "go get"? {#get_version}
 
 The Go toolchain has a built-in system for managing versioned sets of related packages, known as *modules*.