Use the standard expression for the availability statement for alarm().

Doc/lib/libsignal.tex

@@ -103,8 +103,9 @@ The \module{signal} module defines the following functions:
   If \var{time} is zero, no alarm id scheduled, and any scheduled
   alarm is canceled.  The return value is the number of seconds
   remaining before a previously scheduled alarm.  If the return value
-  is zero, no alarm is currently scheduled.  Not on Windows.  (See the
-  \UNIX{} man page \manpage{alarm}{2}.)
+  is zero, no alarm is currently scheduled.  (See the \UNIX{} man page
+  \manpage{alarm}{2}.)
+  Availability: \UNIX.
 \end{funcdesc}
 
 \begin{funcdesc}{getsignal}{signalnum}