Document it's experimental

Author: hynek

src/attr/_funcs.py

@@ -381,7 +381,11 @@ def has(cls):
 
 def inspect(cls):
     """
-    Inspect the class and return it's effective build parameters.
+    Inspect the class and return its effective build parameters.
+
+    Warning:
+
+        This feature is currently **experimental** and is not covered by our strict backwards-compatibility guarantees.
 
     Args:
         cls: The class to inspect.