Add note about type inference.

docs/src/quickstart/cythonize.rst

@@ -136,3 +136,13 @@ and for determining when to :ref:`release the GIL <nogil>`:
 in general, a ``nogil`` block may contain only "white" code.
 
 .. figure:: htmlreport.png
+
+Note that Cython deduces the type of local variables based on their assignments,
+which can also cut down on the need to explicitly specify types everywhere.
+For example, declaring ``dx`` to be of type double above is unnecessary,
+as is declaring the type of ``s`` in the last version (where the return type
+of ``f`` is known to be a C double.)  A notable exception, however, is
+*integer types used in arithmetic expressions*, as Cython is unable to ensure
+that an overflow would not occur (and so falls back to ``object`` in case
+Python's bignums are needed).  To allow inference of C integer types, set the
+``infer_types`` :ref:`directive <compiler-directives>` to ``True``.