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

interface/wx/propgrid/propgrid.h

@@ -942,6 +942,10 @@ public:
         @param editable
             Using @false here will disable column from being editable.
 
+        Note that @a column must not be equal to 1, as the second column is
+        always editable and can be made read-only only on cell-by-cell basis
+        using @code wxPGProperty::ChangeFlag(wxPG_PROP_READONLY, true) @endcode
+
         @see BeginLabelEdit(), EndLabelEdit()
     */
     void MakeColumnEditable( unsigned int column, bool editable = true );

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;