Official documentation (https://docs.openshift.com) is missing "Using an existing Jaeger instance" chapter.

For 1.1 it's described in the maistra docs:

https://maistra.io/docs/monitoring_and_tracing/using-existing-jaeger/

Configuring Jaeger chapter from that docs can be reused.

Configuring the ServiceMeshControlPlane chapter is different for 2.0:
You should just need to specify:
 
spec:
  tracing:
    type: Jaeger # enable tracing, using Jaeger
  addons:
    jaeger:
      name: my-jaeger-resource # name of a pre-existing jaeger resource in the cp namespaec