gopls/doc: document updating to an unstable version
gopls/doc: document updating to an unstable version

Fixes golang/go#36751

Change-Id: Ib5036bcc1f8ee0ab11d9978eea597222e86cc085
Reviewed-on: https://go-review.googlesource.com/c/tools/+/216437
Reviewed-by: Rebecca Stambler <d7cf11e5f299d88ea8348366a9fc4f7c335c3afa@golang.org>
Run-TryBot: Rebecca Stambler <d7cf11e5f299d88ea8348366a9fc4f7c335c3afa@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

golang/tools,golang/tools,golang/tools,golang/tools,golang/tools