Mention that getsid is new in 2.4.
diff --git a/Doc/lib/libos.tex b/Doc/lib/libos.tex
index 624fbf8..a8ceac8 100644
--- a/Doc/lib/libos.tex
+++ b/Doc/lib/libos.tex
@@ -257,7 +257,7 @@
 \begin{funcdesc}{getsid}{pid}
 Calls the system call \cfunction{getsid()}.  See the \UNIX{} manual
 for the semantics.
-Availability: \UNIX.
+Availability: \UNIX. \versionadded{2.4}
 \end{funcdesc}
 
 \begin{funcdesc}{setsid}{}