Relays: explain main use cases and per-protocol limits

[lidel]

Ref. https://docs.libp2p.io/concepts/nat/circuit-relay/

The page should mention that

• relays are limited by default (time/bytes)
• relays only allow specific libp2p protocols to be relays (e.g. ping, identify, hole punching ones)
  • relays do not allow protocols like bitswap by default
• one can run own relay with different rules
  • lift bandwidth / time limits for specific IPs
  • allow extra protocols
  • NOTE: there should be warning this is a potential cost center and implementers should be very careful / have logic ensuring only their "blessed" clients use their "unlimited" relays