docs: review docs around auth & Jaeger

[axw]

#3394 introduces token-based auth support for Jaeger.

Because this is not a feature natively supported by Jaeger, it is opt-in: you have to explicitly specify a process tag to use, and only once you've done that will secret-token or api-key auth be enforced for Jaeger. We should ensure the docs are clear on this.

[axw]

While reviewing the docs I came across a false statement in our Jaeger docs:

If APM Server has apm-server.jaeger.grpc.auth_tag set, it will look for a Process tag of that name in incoming events, and use it for authorizing the Jaeger Agent against the configured secret token or API Keys. The auth tag will not be removed from the events after being verified.

Emphasis mine – that last sentence is incorrect, we do remove those tags.