note about optional

Author: Vladimir Kotal

synchro.tex

@@ -632,6 +632,8 @@
 \end{slide}
 
 \begin{itemize}
+\item The POSIX semaphore APIs are part of the \emph{POSIX Realtime Extension}
+and are optional. For example, macOS supports only named POSIX semaphores.
 \item \emph{value} is the initial semaphore value
 \item Both the \emph{mode} and \emph{value} parameters are optional. This means
 that \emph{mode} has to be specified in order to set the initial value.