renamed the parameter, improved the comment

dumux/material/eos/pengrobinson.hh

@@ -141,20 +141,23 @@ public:
      * \param params Parameters
      * \param phaseIdx The phase index
      * \param isGasPhase Specifies the phase state
-     * \param handleSingleRootSpecially Special handling of the case when the EOS has only one
-     *        intersection with the pressure: the critical molar volume is returned if the
-              fluid is critical, or extrema of the EOS are considered if the fluid is not
-              critical. If the parameter is false and the EOS has only one intersection
-              with, the molar volume is computed from that single intersection, not
-              depending of the given phase (gas or fluid). If the EOS has three
-              intersections with the pressure, this parameter is ignored.
+     * \param handleUnphysicalPhase Special handling of the case when the EOS has only one
+              intersection with the pressure, and the intersection does not correspond to
+              the given phase (the phase is thus considered unphysical). If it happens in
+              the case of critical fluid, the critical molar volume is returned for the
+              unphysical phase. If the fluid is not critical, a proper extremum of the
+              EOS is returned for the unphysical phase. If the parameter is false and the
+              EOS has only one intersection with the pressure, the molar volume is computed
+              from that single intersection, not depending of the given phase (gas or
+              fluid). If the EOS has three intersections with the pressure, this parameter
+              is ignored.
      */
     template <class FluidState, class Params>
     static Scalar computeMolarVolume(const FluidState &fs,
                                      Params &params,
                                      int phaseIdx,
                                      bool isGasPhase,
-                                     bool handleSingleRootSpecially = true)
+                                     bool handleUnphysicalPhase = true)
     {
         Scalar Vm = 0;
 
@@ -204,7 +207,7 @@ public:
             Vm = Z[0]*RT/p;
 
             // Handle single root case specially unless told otherwise
-            if (handleSingleRootSpecially)
+            if (handleUnphysicalPhase)
                 Vm = handleSingleRoot_(Vm, fs, params, phaseIdx, isGasPhase);
         }
         else