Summary

This PR clarifies the transport-level filtering behavior reported in #2388.

What changed

• Added an explicit section to docs/transports.md describing how filtering differs between:
  • single target / single pipeline
  • multiple targets
• Added a clearer ASCII architecture diagram that separates main-thread and worker-thread filtering.
• Clarified in docs/api.md that:
  • targets[i].level defaults to info when omitted
  • top-level level in pino.transport({...}) is not a filter
  • per-target filtering is applied in multi-target mode

Why

Issue #2388 highlights confusion around where and when levels are applied. The current implementation is correct but under-documented. This PR documents the actual architecture and filtering gates to reduce misconfiguration and surprises.

Closes #2388