Improve documentation of wxPropertyGrid::MakeColumnEditable()

Explain that column must be different from 1 in the documentation and in
the (already existing) assert checking it.

Closes https://github.com/wxWidgets/wxWidgets/pull/656

src/propgrid/propgrid.cpp

@@ -964,7 +964,13 @@ void wxPropertyGrid::DoSetSelection( const wxArrayPGProperty& newSelection,
 void wxPropertyGrid::MakeColumnEditable( unsigned int column,
                                          bool editable )
 {
-    wxASSERT( column != 1 );
+    // The second column is always editable. To make it read-only is a property
+    // by property decision by setting its wxPG_PROP_READONLY flag.
+    wxASSERT_MSG
+    (
+         column != 1,
+         wxS("Set wxPG_PROP_READONLY property flag instead")
+    );
 
     wxArrayInt& cols = m_pState->m_editableColumns;