DOCS/man/input: document dummy properties

these properties cannot be used directly and require a
sub-property. document this explicitly.

DOCS/man/input.rst

@@ -3640,6 +3640,8 @@ Property list
     If tracks of the requested type are selected via ``--lavfi-complex``, the
     first one is returned.
 
+    Note that this property cannot be used directly, a sub-property is required.
+
 ``chapter-list`` (RW)
     List of chapters, current entry marked.