[grad.tex] clarify what superscript in parenthesis means

papers/grad.tex

@@ -102,6 +102,8 @@ Let's compute the derivatives of all our models. Throughout the entire paper $n$
   y &= \sigma(a^{(1)}w^{(2)} + b^{(2)})
 \end{align}
 
+The superscript in parenthesis denotes the current layer. For example $a_i^{(l)}$ denotes the activation from the $l$-th layer on $i$-th sample.
+
 \subsubsection{Feed-Forward}
 
 \begin{align}