Telemetry: Agent configuration - Update the documentation

[o0Ignition0o]

Followup to #3950, the tracing configuration documentation is out of date:

trace_config doesn't exist anymore since #4044

This image claims jaeger: agent: endpoint takes a socket address, yet shows a hostname+port, which the router won't allow.

As a side effect of us using serde untagged enums, errors raised while trying to deserialize to a variant greet the user with a difficult to diagnose error message:

plugin apollo.telemetry could not be configured: data did not match any variant of untagged enum Config

[oliverjumpertz]

This has actually already been solved by @BrynCooke with #3962 :)

Just checked when I wanted to quickly fix the docs.

[abernix]

Thanks for pointing that out, @oliverjumpertz! Closing as it's already been fixed! (Thanks, @BrynCooke , too. ;) )