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/msw/registry.h

@@ -112,7 +112,7 @@ public:
     HKUSR, ///< HKEY_USERS
     HKPD,  ///< HKEY_PERFORMANCE_DATA
     HKCC,  ///< HKEY_CURRENT_CONFIG
-    HKDD,  ///< HKEY_DYN_DATA (Obsolete: Windows 9x only)
+    HKDD,  ///< HKEY_DYN_DATA (Obsolete)
     HKMAX
     };