Updated doc following code review.

doc/library/config.txt

@@ -127,12 +127,12 @@ import theano and print the config variable, as in:
     Default: ``False``
 
     If ``True`` and ``device=gpu*``, we raise an error if we cannot
-    use the specified :attr:`device`. If ``True`` and ``device=cpu``, we disable
-    the GPU.  If ``False``, we warn and fall back to the CPU if ``device=gpu*``
-    and can't use the GPU.
+    use the specified :attr:`device`. If ``True`` and ``device=cpu``,
+    we disable the GPU.  If ``False`` and ``device=gpu*``, and if the
+    specified device cannot be used, we warn and fall back to the CPU.
 
     This is useful to run Theano's tests on a computer with a GPU, but
-    to don't run GPU's tests.
+    without running the GPU tests.
 
     This flag's value cannot be modified during the program execution.