Improve wxUSE_STD_IOSTREAM documentation

Clarify that it is 1 by default in the documentation, instead of
implying that it's off by default and needs to be enabled.

Improve the description of this option in setup.h files.

And also document this compilation option in the summary page.

docs/doxygen/mainpages/const_wxusedef.h

@@ -221,6 +221,7 @@ library:
 @itemdef{wxUSE_STATUSBAR, Use wxStatusBar class.}
 @itemdef{wxUSE_STC, Use wxStyledTextCtrl.}
 @itemdef{wxUSE_STDPATHS, Use wxStandardPaths class.}
+@itemdef{wxUSE_STD_IOSTREAM, Use standard stream classes in addition to or, in wxDocument, instead of, wx streams.}
 @itemdef{wxUSE_STOPWATCH, Use wxStopWatch class.}
 @itemdef{wxUSE_STREAMS, Enable stream classes.}
 @itemdef{wxUSE_SVG, Use wxSVGFileDC class.}