add explicit section about literal prefixes to strings tutorial (and …

…mention PEP 498 in it)

docs/src/tutorial/strings.rst

@@ -55,10 +55,25 @@ Python 2 for backwards compatibility reasons.  It is not compatible with
 the :obj:`bytes` type.  Its usage should be rare in normal Cython code as
 the generic :obj:`object` type (i.e. untyped code) will normally be good
 enough and has the additional advantage of supporting the assignment of
-string subtypes.  Support for the :obj:`basestring` type is new in Cython
+string subtypes.  Support for the :obj:`basestring` type was added in Cython
 0.20.
 
 
+String literals
+---------------
+
+Cython understands all Python string type prefixes:
+
+* ``b'bytes'`` for byte strings
+* ``u'text'`` for Unicode strings
+* ``f'formatted {value}'`` for formatted Unicode string literals as defined by
+  `PEP 498 <https://www.python.org/dev/peps/pep-0498/>`_ (added in Cython 0.24)
+
+Unprefixed string literals become :obj:`str` objects when compiling
+with language level 2 and :obj:`unicode` objects (i.e. Python 3
+:obj:`str`) with language level 3.
+
+
 General notes about C strings
 -----------------------------