debug: document when build/test flags are applied vs ignored

[polinasok]

There is been some confusion as to which flags apply when and which ones are ignored (e.g. #1636, example, #855). This is prompting me to suggest that we clearly document these interactions in the following places:

• package.json description for each flag
  • go.delveConfig already says "Debug configuration in the launch.json file will override these values", we should make such comments standard, possibly in both directions (for launch.json attributes - "Overrides xyz in settings.json" and for setting.json - "Will be overriden by xyz in launch.json")
  • must also clarify when these do not apply to codelens options to run/test
• overview docs like https://github.com/golang/vscode-go/blob/master/docs/debugging.md and https://github.com/golang/vscode-go/blob/master/docs/settings.md
• tests (best way to keep the knowledge we need to document up-to-date)

[polinasok]

Related issues: #128, #1027, #855, #1636, #1831