reword in intro

doc/introduction.txt

@@ -14,16 +14,16 @@ The term "mathematical expressions" is used broadly to mean a
 computation with some inputs, possibly which might update the inputs
 in-place. Neural net forward propagation is an expression, as is fprop +
 bprop + a weight-update implementing stochastic gradient descent. Feature
-extraction of zero-crossing, 16 mfcc, and 128 rceps coefficients is also
-an expression.
+extraction from audio of zero-crossing, 16 mfcc, and 128 rceps coefficients is
+also an expression.
 
 Theano melds some aspects of a computer algebra system (CAS) with
 aspects of an optimizing compiler. It can even transform some or
 all of the mathematical expression into C code and compile it into
 native machine instructions. This combination of CAS with optimizing
 compilation is particularly useful for computational fields in which
-complicated mathematical expressions are evaluated numerous times over
-large data sets.
+complicated mathematical expressions are evaluated repeatedly and evaluation
+speed is critical.
 
 Theano supports a range of numerical types in multiple dimensions and
 a number of well-tested operations. It also allows you to compute the