docs(ad): polish Lux example comments + retrigger Doc CI

[ChrisRackauckas-Claude]

Note: Please ignore this PR until reviewed by @ChrisRackauckas. Opening as a draft.

Summary

Tiny wording tweak to the Lux/AD example in docs/src/examples/ad.md:

• Tightened the comment describing the Dense(1 => 2) model.
• Added a # reproducibility hint next to the Random.seed! call.

Why a follow-up PR

PR #442 (Flux → Lux + Optimisers + Zygote) was merged on 2026-04-30
with a failing Documentation CI run. The failure was a package
resolution error, not anything in the example code:

ERROR: Unsatisfiable requirements detected for package MLDataDevices [7e8f7934]:
  - restricted by compatibility requirements with RecursiveArrayTools [731186ca]
    to versions: uninstalled
  - restricted by compatibility requirements with Lux [b2108857]
    to versions: 1.5.0 - 1.17.8 — no versions left

Lux 1.x required MLDataDevices ≤ 1.17.8, but RecursiveArrayTools v4
(pinned by DiffEqGPU) needed a newer MLDataDevices. As of this week
that's been unblocked upstream — MLDataDevices 1.17.10 resolves
cleanly with RecursiveArrayTools 4.3.0 and Lux 1.31.4.

This PR exists primarily to retrigger the Doc CI on a current
registry snapshot now that the resolver conflict is gone.

Test plan

• Pkg.instantiate on docs/Project.toml resolves locally with
  the post-unblock registry — 81 deps precompile cleanly.
• Lux training loop in the example runs end-to-end on CPU
  (EnsembleSerial):

  loss(ps) = 42.0898
  loss(ps) = 9.875094
  loss(ps) = 5.4691095
  ...
  

• Doc CI passes on this branch.
• Runic format-check passes.