Add an availability statement for alarm().

Doc/lib/libsignal.tex

@@ -105,6 +105,7 @@ The \module{signal} module defines the following functions:
   remaining before a previously scheduled alarm.  If the return value
   is zero, no alarm is currently scheduled.  (See the \UNIX{} man page
   \manpage{alarm}{2}.)
+  Availability: \UNIX.
 \end{funcdesc}
 
 \begin{funcdesc}{getsignal}{signalnum}