clarify GitHub ext svc docs, standardize on terms
Created by: sqs
- fix #2946 (closed) by implying that no "repo" scope is needed in the example JSON and explicitly mentioning it in the JSON Schema docs (the explicit mention is omitted from the example JSON for brevity, so that the "token" property is above the fold on an average-sized screen)
- use "repository" not "repo" per https://docs.sourcegraph.com/dev/style_guide