Document killpg.
diff --git a/Doc/lib/libos.tex b/Doc/lib/libos.tex
index 2e0f8ae..42acc35 100644
--- a/Doc/lib/libos.tex
+++ b/Doc/lib/libos.tex
@@ -1120,6 +1120,14 @@
Availability: \UNIX.
\end{funcdesc}
+\begin{funcdesc}{killpg}{pgid, sig}
+\index{process!killing}
+\index{process!signalling}
+Kill the process group \var{pgid} with the signal \var{sig}.
+Availability: \UNIX.
+\versionadded{2.3}
+\end{funcdesc}
+
\begin{funcdesc}{nice}{increment}
Add \var{increment} to the process's ``niceness''. Return the new
niceness.