Remove obsolete symbols and platforms from the documentation

Don't mention pre-XP Windows versions and other obsolete platforms such as
OS/2 and the symbols which were only useful for them.

Closes https://github.com/wxWidgets/wxWidgets/pull/137

interface/wx/filedlg.h

@@ -22,7 +22,7 @@ enum
 /**
     Default wildcard string used in wxFileDialog corresponding to all files.
 
-    It is defined as "*.*" under MSW and OS/2 and "*" everywhere else.
+    It is defined as "*.*" under MSW and "*" everywhere else.
 */
 const char wxFileSelectorDefaultWildcardStr[];