add section about bundlers gotchas to the Troubleshooting docs

See https://discuss.elastic.co/t/how-do-i-setup-log-correlation-when-using-extension-layers/320309/7 for some motivation.
While we have a number of "Gotchas" described here: https://www.elastic.co/guide/en/apm/agent/nodejs/current/starting-the-agent.html#start-gotchas
those same gotchas aren't always just about *starting* the agent. Some refactoring of docs and/or a small section in the Troubleshooting docs pointing to these would be helpful.

(It would also be really nice if the APM agent could detect and log something when it notices it has been bundled, and hence is broken. Partly this is also about possibly having automatic hand-off to a non-bundled agent from a bundled agent -- via better global singleton Agent handling.)