Every example is dimensional, or says plainly that it is not

[lmoresi]

A number in a geodynamics script means nothing without its unit. A reader who
cannot tell whether 1e-6 is a diffusivity in m²/s or a non-dimensional group
can neither check the result nor reuse the script.

So an example either carries units:

kappa = uw.Param(1.0e-6, units="m**2/s", description="thermal diffusivity")

or states its scaling where the parameters are set, naming what was scaled by
what:

# Non-dimensional throughout: lengths by the layer depth D, time by D^2/kappa,
# temperature by the drop across the layer. Ra is then the only free number.

Not both, and never neither.

Tutorials are the exception, deliberately: they teach one idea at a time,
and units there can be the idea getting in the way.

This sits beside the existing clause pointing at Underworld3's own guides —
uw.Param is already units-aware, so for most examples this is a units=
argument rather than new machinery.

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