Document which onion service stats include v3 onion services
We should document which metrics stats include v2 and v3 onion services:
The unique onion addresses stat is v2 only: https://metrics.torproject.org/hidserv-dir-onions-seen.html (See #23126 (moved) for a ticket to add v3 support)
But the onion service traffic includes v2 and v3: https://metrics.torproject.org/hidserv-rend-relayed-cells.html (See #24047 (moved) for a ticket to split the total into v2 and v3)