Additional migration notes from raven-js (#2629)
Additional migration notes from raven-js (#2629)

A few important keys were renamed (such as `shouldSendCallback`) without
any notice in the docs.

Additionally, this commit adds links to the appropriate documentation pages
so the user can learn more about how to use new features, such as `withScope`.
getsentry/raven-js,getsentry/raven-js,getsentry/raven-js,getsentry/raven-js