document 'language_level' compiler directive

docs/src/reference/compilation.rst

@@ -204,6 +204,16 @@ Cython code.  Here is the list of currently supported directives:
     None, indicating that on safe (semantically-unchanging) inferences
     are allowed.
 
+``language_level`` (2/3)
+    Globally set the Python language level to be used for module
+    compilation.  Default is compatibility with Python 2.  To enable
+    Python 3 source code semantics, set this to 3 at the start of a
+    module or pass the "-3" command line option to the compiler.
+    Note that cimported and included source files inherit this
+    setting from the module being compiled, unless they explicitly
+    set their own language level.
+
+
 How to set directives
 ---------------------