Document optional kwargs argument to start_new_thread(). Also

document new LockType and reverse the preference for exit()
vs. exit_thread() -- exit() is now preferred and exit_thread() is
obsolete.

Doc/lib/libthread.tex

@@ -24,21 +24,27 @@ It defines the following constant and functions:
 Raised on thread-specific errors.
 \end{excdesc}
 
-\begin{funcdesc}{start_new_thread}{func, arg}
+\begin{datadesc}{LockType}
-Start a new thread.  The thread executes the function \var{func}
+This is the type of lock objects.
-with the argument list \var{arg} (which must be a tuple).  When the
+\end{datadesc}
+
+\begin{funcdesc}{start_new_thread}{function, args\optional{kwargs}}
+Start a new thread.  The thread executes the function \var{function}
+with the argument list \var{args} (which must be a tuple).  The
+optional \var{kwargs} argument specifies a dictionary of keyword
+arguments.  When the
 function returns, the thread silently exits.  When the function
 terminates with an unhandled exception, a stack trace is printed and
 then the thread exits (but other threads continue to run).
 \end{funcdesc}
 
 \begin{funcdesc}{exit}{}
-This is a shorthand for \function{exit_thread()}.
+Raise the \exception{SystemExit} exception.  When not caught, this
+will cause the thread to exit silently.
 \end{funcdesc}
 
 \begin{funcdesc}{exit_thread}{}
-Raise the \exception{SystemExit} exception.  When not caught, this
+This is an obsolete synonym for \function{exit()}.
-will cause the thread to exit silently.
 \end{funcdesc}
 
 %\begin{funcdesc}{exit_prog}{status}