Edit docstring

theano/scalar/basic_scipy.py

@@ -87,11 +87,14 @@ erfc = Erfc(upgrade_to_float_no_complex, name='erfc')
 
 class Erfcx(UnaryScalarOp):
     """
-    Implements the scaled complementary error function exp(x**2)*erfc(x) in a numerically stable way.
+    Implements the scaled complementary error function exp(x**2)*erfc(x) in a numerically stable way for large x. This
+    is useful for calculating things like log(erfc(x)) = log(erfcx(x)) - 2 * x without causing underflow. Should only
+    be used if x is known to be large and positive, as using erfcx(x) for large negative x may instead introduce
+    overflow problems.
 
     Note: This op can still be executed on GPU, despite not having c_code.  When
     running on GPU, sandbox.cuda.opt.local_gpu_elemwise_[0,1] replaces this op
-    with sandbox.cuda.elemwise.ErfcxGPU. (Same as Erfinv below.)
+    with sandbox.cuda.elemwise.ErfcxGPU.
     """
     def impl(self, x):
         if imported_scipy_special: