[transport] addressing review comments

doc/sphinx/reference/transport/index.md

@@ -52,7 +52,7 @@ High-pressure Gas
   phase entry to `high-pressure`. Implemented by class {ct}`HighPressureGasTransport`.
 
 Chung high-pressure Gas
-: A model for high-pressure gas transport properties that usesthe method of Chung
+: A model for high-pressure gas transport properties that uses the method of Chung
   {cite:p}`poling2001` for viscosity and thermal conductivity and the Takahashi method
   {cite:p}`takahashi1975,poling2001` for the diffusion coefficient. This high-pressure
   gas transport model can be specified in the YAML format by setting the

include/cantera/transport/GasTransport.h

@@ -54,12 +54,20 @@ class GasTransport : public Transport
         std::copy(m_visc.begin(), m_visc.end(), visc);
     }
 
-    //! Returns the matrix of binary diffusion coefficients.
-    /*!
-     * d[ld*j + i] = rp * m_bdiff(i,j);
+    /**
+     * Computes the matrix of binary diffusion coefficients. Units are m^2/s.
+     *
+     * The matrix is dimension m_nsp x m_nsp, where m_nsp is the number of
+     * species. The matrix is stored in row-major order, so that d[ld*j + i]
+     * contains the binary diffusion coefficient of species i with respect to
+     * species j.
+     *
+     * d[ld*j + i] = m_bdiff(i,j) / p
      *
-     * @param ld   offset of rows in the storage
-     * @param d    output vector of diffusion coefficients. Units of m**2 / s
+     * @param ld   Inner stride for writing the two dimension diffusion
+     *             coefficients into a one dimensional vector
+     * @param d    Diffusion coefficient matrix (must be at least m_nsp * m_nsp
+     *             in length.
      */
     void getBinaryDiffCoeffs(const size_t ld, double* const d) override;