docs: clarify WSL daemon socket fallback

[zengwenliang416]

Summary

• Update the README troubleshooting note for WSL2 /mnt projects after the daemon socket fallback work.
• Clarify that current builds relocate the shared daemon socket to the system temp directory when the project filesystem cannot host it.
• Keep CODEGRAPH_NO_DAEMON=1 documented as the remaining fallback if daemon connections still drop.

Test plan

• git diff --check

Docs-only change.