DOC: Add B23 equation in doc.

documentation/sphinx/source/iapws_theory/iapws_r797.rst

@@ -33,4 +33,18 @@ and the molar mass of ordinary water [5, 6]. The values of the critical paramete
 * :math:`p_c = 22.064\ MPa` (3) 
 * :math:`\rho _c = 322\ kg.m^{-3}` (4)
 
-are from the corresponding IAPWS release [7].
+are from the corresponding IAPWS release [7].
+
+
+Auxiliary Equation for the Boundary between Regions 2 and 3
+-------------------------------------------------------------
+
+.. math::
+ :label: eq_B23_P
+
+ \pi = n_1 + n_2 \theta + n_3 \theta ^2
+
+.. math::
+ :label: eq_B23_T
+
+ \theta = n_4 + \left( \frac{\pi - n_5}{n_3} \right)^{1/2}