PoW FAQ update

Description

Given the recent developments on Proof Of Work for Onion Services, it's FAQ needs an update.

Add an admonition that this feature is still experimental. While we encourage operators to try it out, it may contain bugs and may not be effective under specific situations. Operators are advised to try and use the implementation that best suits their use cases, and give feedback in case of issues.
Status updates, as per discussion on 2025-08-07:
- On the Arti implementation.
- On the C Tor implementation.
Inform that implementations differ: as of 2025-08, C Tor and Arti implementations are based on distinct control loop algorithms. Point to the relevant Tor specifications.