add some USR1/USR2 examples

Author: vladak

signals.tex

@@ -320,7 +320,8 @@
 \end{verbatim}
 \item \texttt{SIGTERM} is the default signal for command \texttt{kill(1)}.
 \item \texttt{SIGUSR1} and \texttt{SIGUSR2} are not used by any system call and
-are available for use to the user.
+are available for use to the user. These are used to e.g. dump internal state
+of a daemon to a file, display statistics of the program run so far, etc.
 \end{itemize}
 
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