Understand typing.Optional in annotations (#14397)

crates/red_knot_python_semantic/resources/mdtest/annotations/optional.md

@@ -0,0 +1,47 @@
+# Optional
+
+## Annotation
+
+`typing.Optional` is equivalent to using the type with a None in a Union.
+
+```py
+from typing import Optional
+
+a: Optional[int]
+a1: Optional[bool]
+a2: Optional[Optional[bool]]
+a3: Optional[None]
+
+def f():
+    # revealed: int | None
+    reveal_type(a)
+    # revealed: bool | None
+    reveal_type(a1)
+    # revealed: bool | None
+    reveal_type(a2)
+    # revealed: None
+    reveal_type(a3)
+```
+
+## Assignment
+
+```py
+from typing import Optional
+
+a: Optional[int] = 1
+a = None
+# error: [invalid-assignment] "Object of type `Literal[""]` is not assignable to `int | None`"
+a = ""
+```
+
+## Typing Extensions
+
+```py
+from typing_extensions import Optional
+
+a: Optional[int]
+
+def f():
+    # revealed: int | None
+    reveal_type(a)
+```