gopls/doc/advanced.md: update unstable version installation instruction

`go get` no longer builds/installs binaries in tip.

Updates golang/go#46880

Change-Id: I5673368a579d79f571adf5ca5c44c15b8101dbe9
Reviewed-on: https://go-review.googlesource.com/c/tools/+/354169
Trust: Hyang-Ah Hana Kim <[email protected]>
Reviewed-by: Rebecca Stambler <[email protected]>

Author: hyangah

gopls/doc/advanced.md

@@ -19,7 +19,17 @@ Where `vX.Y.Z` is the desired version.
 To update `gopls` to the latest **unstable** version, use:
 
 ```sh
-GO111MODULE=on go get golang.org/x/tools/gopls@master golang.org/x/tools@master
+# Create an empty go.mod file, only for tracking requirements.
+cd $(mktemp -d)
+go mod init gopls-unstable
+
+# Use 'go get' to add requirements and to ensure they work together.
+go get golang.org/x/tools/gopls@master golang.org/x/tools@master
+
+# For go1.17 or older, the above `go get` command will build and
+# install `gopls`. For go1.18+ or tip, run the following command to install
+# using selected versions in go.mod.
+go install golang.org/x/tools/gopls
 ```
 
 ## Working on the Go source distribution
@@ -55,11 +65,11 @@ build gopls at tip. External APIs are under active development on the
 Go `master` branch, so building gopls at tip minimizes the chances of
 a build failure.
 
-```
-$ GO111MODULE=on gotip get golang.org/x/tools/gopls@master golang.org/x/tools@master
-```
+Build and install the latest **unstable** version of `gopls` following
+[the instruction](#installing-unreleased-versions).
+Remember to use `gotip` instead of `go`.
 
-This will build a version of gopls that understands generic code. To actually
+The `gopls` build with this instruction understands generic code. To actually
 run the generic code you develop, you must also use the tip version of the Go
 compiler. For example: