[numerics.sat.func] Simplify specification of div_sat.

We know precisely when saturation happens, so we can say that directly.

Author: tkoeppe

source/algorithms.tex

@@ -10847,14 +10847,11 @@
 \expects
 \tcode{y != 0} is \tcode{true}.
 
-\pnum
-Let \tcode{q} be \tcode{x / y}, with any fractional part discarded.
-
 \pnum
 \returns
-If \tcode{q} is representable as a value of type \tcode{T}, \tcode{q};
-otherwise, either the largest or smallest representable value of type \tcode{T},
-whichever is closer to the value of \tcode{q}.
+If \tcode{T} is a signed integer type
+and \tcode{x == numeric_limits<T>::min() \&\& y == -1} is \tcode{true},
+\tcode{numeric_limits<T>::max()}, otherwise, \tcode{x / y}.
 
 \pnum
 \remarks