Add Doc Comment For RegistryMetadata._init_object

[biniona]

Currently, RegistryMetadata._init_object has no explanation of what it is trying to do. In broad strokes, method recursively resolves the bindings Deferred bindings attached to the object being initialized, however there may be some subtlety here that I missed or forgot about.

This issue captures the task of determining exactly what this method does, documenting what it does, and documenting any relevant differences between _init_object and _ainit_object.

This Issue is dependent on #49.