sw: document SwUndoReRead

I.e. the primary use-case is not to re-read the same image, but to read
a different Graphic and set it on an existing SwGrfNode.

Change-Id: I0947e714fc1b3de870b77aaa2b6682707ccec4e5
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/143414
Tested-by: Jenkins
Reviewed-by: Miklos Vajna <vmiklos@collabora.com>
diff --git a/sw/source/core/inc/UndoInsert.hxx b/sw/source/core/inc/UndoInsert.hxx
index 7602820..348cc57 100644
--- a/sw/source/core/inc/UndoInsert.hxx
+++ b/sw/source/core/inc/UndoInsert.hxx
@@ -136,6 +136,7 @@ private:
    std::unique_ptr<Impl> m_pImpl;
};

/// Handles the undo/redo of the 'Replace...' context menu item for an image.
class SwUndoReRead final : public SwUndo
{
    std::optional<Graphic> moGraphic;