Harmonize wxXmlDoctype ctor parameter names

Make the names more clear and similar to the names of member variables
to which they correspond and also use them consistently in the
documentation.

This reverts the changes to this interface header done in 5925893eed
(Fix various doxygen issues in the documentation, 2019-10-20).

include/wx/xml/xml.h

@@ -241,10 +241,12 @@ class WXDLLIMPEXP_XML wxXmlDoctype
 {
 public:
     explicit
-    wxXmlDoctype(const wxString& name = wxString(),
-                 const wxString& sysid = wxString(),
-                 const wxString& pubid = wxString())
-                 : m_rootName(name), m_systemId(sysid), m_publicId(pubid)
+    wxXmlDoctype(const wxString& rootName = wxString(),
+                 const wxString& systemId = wxString(),
+                 const wxString& publicId = wxString())
+                 : m_rootName(rootName),
+                   m_systemId(systemId),
+                   m_publicId(publicId)
                  {}
 
     // Default copy ctor and assignment operators are ok.