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/pen.h

@@ -171,7 +171,6 @@ public:
 
         @remarks Different versions of Windows and different versions of other
                  platforms support very different subsets of the styles above
-                 - there is no similarity even between Windows95 and Windows98 -
                  so handle with care.
 
         @see SetStyle(), SetColour(), SetWidth()