Added explanatory comment

[SVN r76885]

doc/reference/algorithms/convert.qbk

@@ -15,6 +15,9 @@
     (where assign is modelled as target := source)]
 
 [include generated/convert_status.qbk]
+[note In this status matrix above: columns are source types
+  and rows are target types. So a box can be converted to a ring, polygon
+  or multi-polygon, but not vice versa.]
     
 [heading Complexity]
 Linear