gopls: add documentation and warnings about ignored settings

[stamblerre]

See the original discussion at microsoft/vscode-go#2965.

A number of VS Code Go settings, including "go.buildTags" are ignored when used with gopls. We should either propagate these settings or offer a warning that they are being ignored.

[gopherbot]

Change https://golang.org/cl/273146 mentions this issue: src/goLanguageServer: propagate go.buildFlags,buildTags to gopls

[gopherbot]

Change https://golang.org/cl/273147 mentions this issue: package.json: mark settings that are not applicable when using gopls

[gopherbot]

Change https://golang.org/cl/279419 mentions this issue: docs/gopls.md: update documentation about gopls block