[DOC] Improve docs on auto-instrumentation

[lucabelluccini]

Warn users the agent will only trace database queries if they're made within the context of a sampled transaction. Typically people use the agent with web applications, measuring the latency of requests, with details of operations such as database queries in those requests.

When not using a framework, the user needs to start a transaction using apm.startTransaction before making the query, and end the transaction after the query has completed.

Ideally, it might be just a warning banner with a pointer to https://www.elastic.co/guide/en/apm/agent/nodejs/current/custom-transactions.html

From https://discuss.elastic.co/t/nodejs-apm-does-not-report-postgres-queries/208984/2

Please review the wording. This is just a doc improvement which I would like the docs team to review and backport

Checklist

• Update documentation

[apmmachine]

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• Start Time: 2022-10-03T16:19:53.766+0000

• Duration: 6 min 24 sec

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