Incorporate Mike's recommendations

Docs/source/Hydrodynamics.rst

@@ -297,11 +297,7 @@ Source Terms
 We now compute explicit source terms for each variable in :math:`\Qb` and
 :math:`\Ub`. The primitive variable source terms will be used to construct
 time-centered fluxes. The conserved variable source will be used to
-advance the solution. To reduce memory usage, we do not include source terms
-for the advected quantities, species, and auxiliary variables in the conserved
-state vector by default. If your application needs external source terms for
-these variables, set `USE_SPECIES_SOURCES=TRUE` when compiling so that space
-will be allocated for them. We neglect reaction source terms since they are
+advance the solution. We neglect reaction source terms since they are
 accounted for in **Steps 1** and **6**. The source terms are:
 
 .. math::
@@ -346,6 +342,14 @@ accounted for in **Steps 1** and **6**. The source terms are:
    S_{{\rm ext},\rho Y_k}
    \end{array}\right)^n.
 
+.. index:: USE_SPECIES_SOURCES
+
+.. note:: To reduce memory usage, we do not include source terms for the
+   advected quantities, species, and auxiliary variables in the conserved
+   state vector by default. If your application needs external source terms for
+   these variables, set `USE_SPECIES_SOURCES=TRUE` when compiling so that space
+   will be allocated for them.
+
 Primitive Forms
 ===============