Document that wxGrid::ShowCellEditControl() does not start editing

This was sufficiently misleading that event our own wxGrid unit tests
used this function in an attempt to start editing a grid cell -- even
though it actually doesn't do it at all.

Unfortunately documenting the surprising semantics of this functions
looks like the best thing we can do because it appears to have always
behaved like this and changing it now to actually show the cell editor
control, i.e. starting to edit the cell, is almost certain to break some
existing code.

interface/wx/grid.h

@@ -2866,8 +2866,14 @@ public:
     /**
         Enables or disables in-place editing of grid cell data.
 
-        The grid will issue either a @c wxEVT_GRID_EDITOR_SHOWN or
+        Enabling in-place editing generates @c wxEVT_GRID_EDITOR_SHOWN and, if
-        @c wxEVT_GRID_EDITOR_HIDDEN event.
+        it isn't vetoed by the application, shows the in-place editor which
+        allows the user to change the cell value.
+
+        Disabling in-place editing does nothing if the in-place editor isn't
+        currently show, otherwise the @c wxEVT_GRID_EDITOR_HIDDEN event is
+        generated but, unlike the "shown" event, it can't be vetoed and the
+        in-place editor is dismissed unconditionally.
     */
     void EnableCellEditControl(bool enable = true);
 
@@ -3249,7 +3255,11 @@ public:
     void SetReadOnly(int row, int col, bool isReadOnly = true);
 
     /**
-        Displays the in-place cell edit control for the current cell.
+        Displays the active in-place cell edit control for the current cell
+        after it was hidden.
+
+        Note that this method does @em not start editing the cell, this is only
+        done by EnableCellEditControl().
     */
     void ShowCellEditControl();