Update docs for Datadog metrics backends
Update docs for Datadog metrics backends

When `DogStatsdMetricsBackend` was introduced in #5230 it was added without
clarifying how to use it, or what the difference was from the existing
`DatadogMetricsBackend`.
This changeset aims to correct that.

Signed-off-by: Mike Fiedler <af44b651315133051dded0e55830cbf76e9bfef9@gmail.com>

looker/sentry,mvaled/sentry,ifduyue/sentry,ifduyue/sentry,looker/sentry,looker/sentry,mvaled/sentry,beeftornado/sentry,mvaled/sentry,mvaled/sentry,ifduyue/sentry,ifduyue/sentry,looker/sentry,beeftornado/sentry,beeftornado/sentry,mvaled/sentry,ifduyue/sentry,mvaled/sentry,looker/sentry