update docs for Converter.lazy attribute

asdf/extension/_converter.py

@@ -34,6 +34,11 @@ class Converter(abc.ABC):
     and return a str, the selected tag (should be one of tags) or
     `None` which will trigger the result of ``to_yaml_tree`` to be
     used to look up the next converter for this object.
+
+    The ``lazy`` attribute is optional. If ``True`` asdf will
+    pass "lazy" objects to the converter. If ``False`` (or not
+    defined) asdf will convert all child objects before calling
+    `from_yaml_tree`.
     """
 
     @classmethod

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<Converter>` - 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
 ================