docs: show JSONL source and line diagnostics

[luantaraschi]

Why

The JSONL adapter reports source names and one-based line numbers, but the public docs do not show the resulting diagnostic.

Scope

• Add one malformed JSONL input example.
• Show the exact JsonlTraceError fields or formatted message.
• Explain that source-specific field mapping stays outside the adapter.
• Add or update a test if the example exposes undocumented behavior.

Done when

The documented output is verified against the current implementation.