Document how to attach data onto a span/transaction

[kahest]

Description

Document how a user can add arbitrary data attributes to a span & transaction.
As almost all SDKs, besides JS, do not yet expose a setAtttribute API, we want to document setData instead, but make sure to call these "data attributes" on the docs already.

See https://github.com/getsentry/projects/issues/524

[krystofwoldrich]

https://docs.sentry.io/platforms/react-native/tracing/instrumentation/custom-instrumentation/#improving-span-data