Document if Sender is fork-safe.

Consider a scenario where:
- master process attaches fluent Sender or logging Handler
- master process forks off a bunch of worker processes
- workers use handler

Will the socket get cloned or reopened?
Does fluent logger expect a response from fluentd?
If it does, can the response be read out by wrong sender process?

Start by tracing the Python Sender and logging Handler behavior across forked worker processes. Determine whether the socket is cloned or reopened, whether fluentd responses are expected, and whether another sender could read them. Done means the fork-safety behavior and usage implications are documented clearly.

索引モデルが issue の本文から書いたものです。