[cmath.syn] Add 'lerp' to list of differences vs. C (#4806)

Author: jensmaurer

source/numerics.tex

@@ -9875,7 +9875,8 @@
 The contents and meaning of the header \libheader{cmath}
 are the same as the C standard library header \libheader{math.h},
 with the addition of
-a three-dimensional hypotenuse function~(\ref{c.math.hypot3}) and
+a three-dimensional hypotenuse function~(\ref{c.math.hypot3}),
+a linear interpolation function\iref{c.math.lerp}, and
 the mathematical special functions described in \ref{sf.cmath}.
 \begin{note}
 Several functions have additional overloads in this document,