Fix descriptions for inherited fields in schema generation

[BenjaminPelletier]

Currently, it appears that descriptions for fields that are inherited from a base class are not rendered into auto-generated schema. For instance:

class Foo(ImplicitDict):
    base_field: str
    """This is a base field."""

class Bar(Foo):
    extra_field: str
    """This is an extra field."""

In this case, rendering the schema definition for Bar will exclude "This is a base field" from the description of base_field. Expected behavior: "This is a base field" is included as the description for base_field in the rendered JSON Schema.