Add star import dedup behavior to supported-behaviors docs

docs/tutorial/supported-behaviors.md

@@ -90,6 +90,81 @@ __all__.extend(["walk"])
 
 ---
 
+## Star Import
+
+When used with `--include-star-import`, unimport can refactor star imports into explicit
+imports by detecting which names are actually used in the code.
+
+**input**
+
+```python
+from os import *
+from json import *
+
+print(getcwd())
+print(JSONEncoder)
+```
+
+**output**
+
+```python
+from os import getcwd
+from json import JSONEncoder
+
+print(getcwd())
+print(JSONEncoder)
+```
+
+#### Duplicate Name Resolution
+
+When multiple star imports export the same name, unimport deduplicates suggestions so
+that only the last import provides each name (matching Python's shadowing semantics).
+This produces correct output in a single pass.
+
+**input**
+
+```python
+from _collections import *
+from collections import *
+
+print(defaultdict)
+```
+
+**output**
+
+```python
+from collections import defaultdict
+
+print(defaultdict)
+```
+
+If the star imports partially overlap, each import keeps only its unique names:
+
+**input**
+
+```python
+from collections import *
+from _collections import *
+
+print(Counter)
+print(defaultdict)
+```
+
+**output**
+
+```python
+from collections import Counter
+from _collections import defaultdict
+
+print(Counter)
+print(defaultdict)
+```
+
+Star import suggestions also respect explicit imports — if a name already has an
+explicit import, star imports won't produce a duplicate for that name.
+
+---
+
 ## Scope
 
 Unimport tries to better understand whether the import is unused by performing scope