docs(iroh-net): Document default relay servers a bit more #10408

Job	Run time
Android Build Only (aarch64-linux-android)	1m 34s
cargo deny	33s
Checking docs	1m 14s
Checking fmt	11s
Android Build Only (armv7-linux-androideabi)	1m 28s
check_semver	5m 44s
clippy_check	2m 57s
codespell	8s
Cross Build Only (x86_64-unknown-freebsd)	1m 8s
Cross Test (i686-unknown-linux-gnu)	3m 33s
Docker Test	2m 7s
Minimal Supported Rust Version	1m 23s
Run network simulations/benchmarks	10m 59s
Build wasm32	36s
CI Test Suite / Tests (ubuntu-latest, stable, all)	4m 14s
CI Test Suite / Tests (windows-latest, stable, all, x86_64-pc-windows-msvc)	4m 7s
CI Test Suite / Tests (ubuntu-latest, stable, none)	3m 16s
CI Test Suite / Tests (windows-latest, stable, none, x86_64-pc-windows-msvc)	4m 6s
CI Test Suite / Tests (ubuntu-latest, stable, default)	3m 27s
CI Test Suite / Tests (windows-latest, stable, default, x86_64-pc-windows-msvc)	4m 4s
CI Test Suite / Tests (macOS-arm-latest, stable, all)	4m 52s
CI Test Suite / Tests (macOS-arm-latest, stable, none)	3m 42s
CI Test Suite / Tests (macOS-arm-latest, stable, default)	4m 26s
	1h 9m 49s

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