[rand.eng.philox] Improve descriptions of $Y$ and $i$

Author: tkoeppe

source/numerics.tex

@@ -3090,9 +3090,9 @@
 \item
 $K$ are keys,
 \item
-$Y$ is a buffer of produced values, and
+$Y$ stores a batch of output values, and
 \item
-$i$ is an index in $Y$ buffer.
+$i$ is an index for an element of the sequence $Y$.
 \end{itemize}
 
 \pnum