Summary

Some descriptive text can be added above and below the arrow text.  

Step by Step

Move the mouse in the area immediately above or below the reaction arrow - the area highlights in light blue:

After clicking inside the blue area, a dialog appears for entering descriptive text, either for the area above or below the reaction arrow depending on which area was clicked.

After clicking OK, the entered text appears above on the reaction arrow:  

In the same manner also descriptive text below the reaction arrow can be entered.