update docs for Converter.lazy attribute

docs/asdf/extending/converters.rst

@@ -56,6 +56,14 @@ when some logic is required to select the tag to assign to a ``to_yaml_tree`` re
 `Converter.select_tag<Converter>` - an optional method that accepts a complex Python object and a list
 candidate tags and returns the tag that should be used to serialize the object.
 
+`Converter.lazy` - a boolean attribute indicating if this converter accepts "lazy" objects
+(those defined in `asdf.lazy_nodes`). This is mostly useful for container-like classes
+(where the "lazy" objects can defer conversion of contained objects until they are accessed).
+If a converter produces a generator lazy should be set to ``False`` as asdf will need
+to generate nodes further out the branch to fully resolve the object returned from the
+generator.
+
+
 A simple example
 ================