x/gools/gopls: document settings for working on source distribution
x/gools/gopls: document settings for working on source distribution

Gopls needs to use the right go command to work properly on a source
distribution. One way of getting this done is by setting go.alternateTools.

Fixes golang/go#38603

Change-Id: Ie379b372c48f651a22c73d93f8608194c5e3a417
Reviewed-on: https://go-review.googlesource.com/c/tools/+/253297
Run-TryBot: Peter Weinberger <dd140fb9530d791057dd8833ea6768ff65fcde71@google.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Rebecca Stambler <d7cf11e5f299d88ea8348366a9fc4f7c335c3afa@golang.org>

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