update docstring for Bound

Author: Junpeng Lao

pymc3/distributions/distribution.py

@@ -476,14 +476,17 @@ class Bound(object):
 
     Example
     -------
-    # In general Bounded distribution need to be defined befor model context
-    boundedNormal = pm.Bound(pm.Normal, lower=0.0)
+    # Bounded distribution can be defined before the model context
+    PositiveNormal = pm.Bound(pm.Normal, lower=0.0)
     with pm.Model():
-        par1 = boundedNormal('par1', mu=0.0, sd=1.0, testval=1.0)
+        par1 = PositiveNormal('par1', mu=0.0, sd=1.0, testval=1.0)
+        # or within the model context
+        NegativeNormal = pm.Bound(pm.Normal, upper=0.0)
+        par2 = NegativeNormal('par2', mu=0.0, sd=1.0, testval=1.0)
         
         # or you can define it implicitly within the model context 
-        par2 = pm.Bound(pm.Normal, lower=0.0)(
-                'par2', mu=0.0, sd=1.0, testval=1.0)
+        par3 = pm.Bound(pm.Normal, lower=-1.0, upper=1.0)(
+                'par3', mu=0.0, sd=1.0, testval=1.0)
     """
 
     def __init__(self, distribution, lower=-np.inf, upper=np.inf):