Reformulation in doc of check_preallocated_output.

doc/library/config.txt

@@ -374,8 +374,9 @@ import theano and print the config variable, as in:
     * ``"previous"``: reuse previously-returned memory,
     * ``"c_contiguous"``: newly-allocated C-contiguous memory,
     * ``"f_contiguous"``: newly-allocated Fortran-contiguous memory,
-    * ``"ALL"``: placeholder for all of the above,
-    * ``""``: Do not test with preallocated memory.
+    * ``"ALL"``: placeholder for all of the above.
+
+    In order not to test with preallocated memory, use an empty string, ``""``.
 
 .. attribute:: config.DebugMode.warn_input_not_reused