Update model equation for hydraulic conductivity (parflow#734)

Added density and gravitational equation. Without those, the units don't
match up. K(p) should be in in LT^-1.
Since \bar k is in L^2, k_r is in [-], and \mu is in ML^-1T^-1, we would
currently be at M^-1LT, which is why we need g in LT^-2 and \rho in
ML^-3. Also, see equation (1) here:
Blake, G.R. et al. (2008). Permeability. In: Chesworth, W. (eds)
Encyclopedia of Soil Science. Encyclopedia of Earth Sciences Series.
Springer, Dordrecht. https://doi.org/10.1007/978-1-4020-3995-9_425

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Co-authored-by: Steven Smith <smithsg84@gmail.com>
Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>
Co-authored-by: Steven Smith <smith84@llnl.gov>

Author: 3 people

docs/user_manual/models.rst

@@ -90,9 +90,13 @@ fluxes). The hydraulic conductivity can be written as,
    :label: hydcond
 
    \begin{aligned}
-   K(p) =  \frac{{\bar k}k_r(p)}{\mu}
+   K(p) =  \frac{{\bar k} k_r(p) \rho g}{\mu},
    \end{aligned}
 
+where :math:`\bar k` is the intrinsic permeability tensor, :math:`k_r(p)` is
+the relative permeability, :math:`\rho` is the density, :math:`g` is the
+gravitational acceleration, and :math:`\mu` is the viscosity.
+
 Boundary conditions can be stated as,
 
 .. math::