Update compilation.rst

docs/src/reference/compilation.rst

@@ -311,6 +311,11 @@ Compiler directives
 Compiler directives are instructions which affect the behavior of
 Cython code.  Here is the list of currently supported directives:
 
+``binding`` (True / False)
+    Controls whether free functions behave more like Python's CFunctions
+    (e.g. `len`) or Python's functions (which, among other things,
+    bind to an instance when looked up as a class attribute).
+
 ``boundscheck``  (True / False)
     If set to False, Cython is free to assume that indexing operations
     ([]-operator) in the code will not cause any IndexErrors to be