Docs: Document Undef in API reference and type stubs

docs/api_reference.rst

@@ -39,6 +39,12 @@ CID classes
     :inherited-members:
     :undoc-members:
 
+.. autoclass:: cid.cid._UndefCID
+    :members:
+    :undoc-members:
+
+.. autodata:: Undef
+
 .. autoclass:: CIDJSONEncoder
     :show-inheritance:
     :members:

newsfragments/68.docs.rst

@@ -0,0 +1 @@
+Document ``Undef`` in the API reference and type stubs.

stubs/cid.pyi

@@ -15,6 +15,7 @@ class BaseCID:
     @property
     def buffer(self) -> bytes: ...
     def encode(self, encoding: str | None = None) -> bytes: ...
+    def defined(self) -> bool: ...
     def __eq__(self, other: object) -> bool: ...
     def __hash__(self) -> int: ...
 
@@ -28,6 +29,16 @@ class CIDv1(BaseCID):
     def __init__(self, codec: str, multihash: str | bytes) -> None: ...
     def to_v0(self) -> CIDv0: ...
 
+class _UndefCID:
+    def defined(self) -> bool: ...
+    def __bool__(self) -> bool: ...
+    def __repr__(self) -> str: ...
+    def __str__(self) -> str: ...
+    def __eq__(self, other: object) -> bool: ...
+    def __hash__(self) -> int: ...
+
+Undef: _UndefCID
+
 # Function overloads for make_cid
 @overload
 def make_cid(base58_encoded_multihash: str) -> CIDv0: ...