add docstring for gammah_rate

Author: mabruzzo

src/clib/rate_functions.c

@@ -1436,11 +1436,10 @@ double comp_rate(double units, chemistry_data *my_chemistry)
     return 5.65e-36 / units;
 }
 
-//Calculation of gammah.
 double gammah_rate(double units, chemistry_data *my_chemistry)
 {
-    //Default is 8.5e-26 for epsilon=0.05, G_0=1.7 (rate in erg s^-1 cm^-3).
     if (my_chemistry->photoelectric_heating <= 1) {
+       // Default: 8.5e-26 for epsilon=0.05, G_0=1.7 (rate in erg s^-1 cm^-3)
         return my_chemistry->photoelectric_heating_rate / units;
     } else { //photoelectric_heating set to 2 or 3
         //User to specify G_0, epsilon set to 0.05 or calculated directly.

src/include/grackle_rate_functions.h

@@ -111,6 +111,13 @@ double regr_rate(double T, double units, chemistry_data *my_chemistry);
 double grain_growth_rate(double T, double units, chemistry_data *my_chemistry);
 
 double comp_rate(double units, chemistry_data *my_chemistry);
+
+/// Calculate quantity closely related to Γ for photo-electric heating from
+/// photo-electric heating by dust-grains
+///
+/// For added context, we currently use equation 1 (and possibly eqn 2) of
+/// [Wolfire+95](https://ui.adsabs.harvard.edu/abs/1995ApJ...443..152W/abstract)
+/// to implement photo-electric heating
 double gammah_rate(double units, chemistry_data *my_chemistry);
 double gamma_isrf_rate(double units, chemistry_data *my_chemistry);
 double gamma_isrf2_rate(double units, chemistry_data *my_chemistry);