Give a slightly better explanation of excepthook.
diff --git a/Doc/lib/libsys.tex b/Doc/lib/libsys.tex
index 44b7853..be5a599 100644
--- a/Doc/lib/libsys.tex
+++ b/Doc/lib/libsys.tex
@@ -51,19 +51,21 @@
\code{sys.displayhook} is called on the result of evaluating
an expression entered in an interactive Python session.
The display of these values can be customized by assigning
-another function to \code{sys.displayhook}.
+another one-argument function to \code{sys.displayhook}.
\end{funcdesc}
\begin{funcdesc}{excepthook}{\var{type}, \var{value}, \var{traceback}}
This function prints out a given traceback and exception to
\code{sys.stderr}.
-\code{sys.excepthook} is called when an exception is raised
-and uncaught. In an interactive session this happens just before
+When an exception is raised and uncaught, the interpreter calls
+\code{sys.excepthook} with three arguments, the exception class,
+exception instance, and a traceback object.
+In an interactive session this happens just before
control is returned to the prompt; in a Python program this happens
just before the program exits.
The handling of such top-level exceptions can be customized by
-assigning another function to \code{sys.excepthook}.
+assigning another three-argument function to \code{sys.excepthook}.
\end{funcdesc}
\begin{datadesc}{__displayhook__}