Ensure the clipboard contents are stored properly on GTK

GTK needs to be told the data on the clipboard can be stored
before the store is attempted.

Also, document that the clipboard behavior on GTK is very
dependent on the configuration of the user's machine.

Closes #10515

samples/clipboard/clipboard.cpp

@@ -151,7 +151,8 @@ void MyFrame::OnFlush(wxCommandEvent &WXUNUSED(event))
         return;
     }
 
-    if ( !wxTheClipboard->AddData(new wxTextDataObject("Text from wx clipboard sample")) )
+    wxString clipData = wxString::Format("Text from wx clipboard sample at %s" , wxDateTime::Now().Format());
+    if ( !wxTheClipboard->AddData(new wxTextDataObject(clipData)) )
     {
         m_textctrl->AppendText("Failed to put text on clipboard.\n");
         return;