[rand.eng.philox] Move description of the key sequence $K$ up to its first mention

Author: tkoeppe

source/numerics.tex

@@ -3088,7 +3088,8 @@
 $X$ is the interpretation of the unsigned integer \term{counter} value
 $Z \cedef \sum_{j = 0}^{n - 1} X_j \cdot 2^{wj}$ of $n \cdot w$ bits,
 \item
-$K$ are keys,
+$K$ are keys, which are generated once from the seed (see constructors below)
+and remain constant unless the \tcode{seed} function\iref{rand.req.eng} is invoked,
 \item
 $Y$ stores a batch of output values, and
 \item
@@ -3169,10 +3170,7 @@
   $\mathit{key}^q_k \cedef (K_k + q \cdot C_k) \mod 2^w$,
 
   \item
-  %FIXME: $K_k$ is just one element - do we mean for each possible $k$?
-  %FIXME: Same for the next 2 \items.
-  $K_k$ are the keys generated once with a seed and stay constant
-  unless the \tcode{seed} function is called,
+  $K_k$ are the elements of the key sequence $K$,
 
   \item
   $M_k$ is \tcode{multipliers[$k$]}, and