setExtras shallow merges objects (doc)

[ptim]

Hi! Thanks for Toucan 🙏

I've found that setExtras shallow merges objects - and on investigation, confirmed that this is also the case in @sentry/core: https://github.com/getsentry/sentry-javascript/blob/master/packages/hub/src/hub.ts#L313

I tested this in a fork, and proposed a changed to the Toucan docs, but this issue may be enough - merging the PR might be unnecessary - whatever!

Test and suggested readme change: ptim/toucan-js/pull/1