docs: warn about using Session across forked processes

[yuchengpersonal]

Context: #4323 notes that creating a Session before forking can lead to unsafe behavior because connection pools may be shared between processes.

This PR adds a short warning to the "Session Objects" section of the advanced usage docs to help users avoid this pitfall.

What changed

• Add a warning admonition about fork()/multiprocessing and Session connection pools

How to verify

• Docs build should succeed (RST-only change)

Risk

• Low; documentation-only change.