docs/settings.md: add description of object type settings' properties

Change-Id: Iaafe9b79461df2f0793d4c041a46d3063bd58d18
Reviewed-on: https://go-review.googlesource.com/c/vscode-go/+/278354
Trust: Hyang-Ah Hana Kim <[email protected]>
Run-TryBot: Hyang-Ah Hana Kim <[email protected]>
TryBot-Result: kokoro <[email protected]>
Reviewed-by: Rebecca Stambler <[email protected]>

Author: hyangah