Fixed grammar error and improved description.

doc/extending/ctype.txt

@@ -132,7 +132,7 @@ contains code that should be inserted wherever an error is identified.
 
 ``c_declare`` and ``c_extract`` also accept a third ``check_input`` 
 optional argument. If you want your type to validate its inputs, it must
-do it only when ``check_input`` is True.
+only do it when ``check_input`` is True.
 
 The example code below should help you understand how everything plays
 out:

doc/tutorial/extending_theano.txt

@@ -136,8 +136,8 @@ The :func:`R_op` method is needed if you want ``theano.tensor.Rop`` to
 work with your op.
 
 The optional boolean :func:'check_input' attribute is used to specify if you want the types used in 
-your op to check their inputs in their c_code. It can be used to speed up compilation and reduce the 
-number of generated c_file.
+your op to check their inputs in their c_code. It can be used to speed up compilation, reduce overhead 
+(particularly for scalars) and reduce the number of generated C files.
 
 Op Example
 ==========