Documentation fix for Slider.step property: (kivy#7305)

fixed wrongly documented default value and added a sentence for zero value behaviour.

kivy/uix/slider.py

@@ -112,11 +112,13 @@ class Slider(Widget):
     .. versionadded:: 1.4.0
 
     Determines the size of each interval or step the slider takes between
-    min and max. If the value range can't be evenly divisible by step the
-    last step will be capped by slider.max
+    :attr:`min` and :attr:`max`. If the value range can't be evenly
+    divisible by step the last step will be capped by slider.max.
+    A zero value will result in the smallest possible intervals/steps,
+    calculated from the (pixel) position of the slider.
 
     :attr:`step` is a :class:`~kivy.properties.NumericProperty` and defaults
-    to 1.'''
+    to 0.'''
 
     background_horizontal = StringProperty(
         'atlas://data/images/defaulttheme/sliderh_background')