docs: Fix stale IPv6 platform claims in rustdoc

[dweekly]

Fixes rustdoc claims that predate the v0.9/v0.10 IPv6 rollout:

• PreferredFamily (config.rs): the Auto rationale said IPv6 probing was "currently macOS-only" and cited platforms without v6 support — both obsolete since v0.10.0 completed Windows. Rewritten: v4-first is kept purely for behavioral compatibility with pre-0.9 traces, and explicitly noted as not-Happy-Eyeballs.
• Ipv6NotSupported doc comments in error.rs, traceroute.rs, and the lib.rs example: clarified this now only fires on non-tier-1 platforms.

Doc comments only — no #[error] message strings or behavior changed. Two things noticed but deliberately left alone: the #[error("IPv6 targets are not yet supported")] runtime string is also slightly stale, and ProviderError::UnsupportedIpVersion (public_ip/providers.rs) is defined but never constructed (removing it is semver-breaking). Both are separate conversations.

Verified: cargo fmt, clippy --all-targets -D warnings, cargo doc with -Dwarnings, and all 28 doctests pass.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.31%. Comparing base (357ca3d) to head (f73543b).

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #55   +/-   ##
=======================================
  Coverage   84.31%   84.31%           
=======================================
  Files          36       36           
  Lines        6663     6663           
=======================================
  Hits         5618     5618           
  Misses       1045     1045           

Flag	Coverage Δ
unittests	84.31% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
src/lib.rs	78.33% <ø> (ø)
src/traceroute.rs	100.00% <ø> (ø)
src/traceroute/config.rs	93.56% <ø> (ø)

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