sw: document SwArrowPortion

One would assume it's just a painted indicator, but no, it has its own
portion.

Change-Id: Iddd93541766cb13600a978465937b1ef7d7e6200
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/119374
Reviewed-by: Miklos Vajna <vmiklos@collabora.com>
Tested-by: Jenkins
diff --git a/sw/source/core/text/porrst.hxx b/sw/source/core/text/porrst.hxx
index c92c50d..9bc1efb 100644
--- a/sw/source/core/text/porrst.hxx
+++ b/sw/source/core/text/porrst.hxx
@@ -97,6 +97,7 @@ public:
    virtual void Paint( const SwTextPaintInfo &rInf ) const override;
};

/// Indicator that the content does not fit into a fixed height frame (red triangle on the UI).
class SwArrowPortion : public SwLinePortion
{
    Point m_aPos;