[FIX] Add docstrings for constants

pulse2percept/utils/constants.py

@@ -1,17 +1,17 @@
 """`DT`, `MIN_AMP`, `ZORDER`"""
 
-# Pulses with net currents smaller than 10 picoamps are considered
-# charge-balanced (here expressed in microamps):
+#: Pulses with net currents smaller than 10 picoamps are considered
+#: charge-balanced (here expressed in microamps).
 MIN_AMP = 1e-5
 
-# Sampling time step (ms); defines the duration of the signal edge
-# transitions:
+#: Sampling time step (ms); defines the duration of the signal edge
+#: transitions.
 DT = 1e-3
 
 
-# An enum specifying the zorder values to use in Matplotlib plots, ensuring
-# that foreground items (like implants) always appear on top of background
-# items (like axon maps):
+#: An enum specifying the zorder values to use in Matplotlib plots, ensuring
+#: that foreground items (like implants) always appear on top of background
+#: items (like axon maps).
 ZORDER = {
     'front': 9999,
     'back': 0,