Update config documentation for magma

doc/library/config.txt

@@ -679,6 +679,29 @@ import theano and print the config variable, as in:
     ``'guess_once'``, ``'guess_on_shape_change'``, ``'time_once'``,
     ``'time_on_shape_change'``.
 
+.. attribute:: config.magma.enabled
+
+    String value: ``'True'``, ``'False'``
+
+    Default: ``'False'``
+
+    If ``'True'``, use `magma <http://icl.cs.utk.edu/magma/>`_ for matrix
+    computations.
+
+    If ``'False'``, disable magma.
+
+.. attribute:: config.magma.include_path
+
+    Default: ``''``
+
+    Location of the magma headers.
+
+.. attribute:: config.magma.library_path
+
+    Default: ``''``
+
+    Location of the magma library.
+
 .. attribute:: config.gcc.cxxflags
 
     Default: ``""``

theano/configdefaults.py

@@ -363,12 +363,12 @@ AddConfigVar('dnn.enabled',
 
 AddConfigVar('magma.include_path',
              "Location of the magma header",
-             StrParam(None),
+             StrParam(''),
              in_c_key=False)
 
 AddConfigVar('magma.library_path',
              "Location of the magma library",
-             StrParam(None),
+             StrParam(''),
              in_c_key=False)
 
 AddConfigVar('magma.enabled',