A little more text, and some really minor cleanups.

Doc/lib/libmain.tex

 \section{\module{__main__} ---
-         Top-level script environment.}
-\declaremodule[main]{builtin}{__main__}
+         Top-level script environment}
 
+\declaremodule[main]{builtin}{__main__}
 \modulesynopsis{The environment where the top-level script is run.}
 
 This module represents the (otherwise anonymous) scope in which the
 interpreter's main program executes --- commands read either from
-standard input or from a script file.
+standard input, from a script file, or from an interactive prompt.  It
+is this environment in which the idiomatic ``conditional script''
+stanza causes a script to run:
+
+\begin{verbatim}
+if __name__ == "__main__":
+    main()
+\end{verbatim}