Bug #839585: mention % string operator in language ref

Doc/ref/ref5.tex

@@ -777,6 +777,12 @@ approximately where \code{x/y} is replaced by \code{floor(x/y)} or
     * y + x \%{} y} be very close to \code{x}.
 }.
 
+In addition to performing the modulo operation on numbers, the \code{\%}
+operator is also overloaded by string and unicode objects to perform
+string formatting (also known as interpolation). The syntax for string
+formatting is described in the Python Library Reference, section
+``Sequence Types''.
+
 \deprecated{2.3}{The floor division operator, the modulo operator,
 and the \function{divmod()} function are no longer defined for complex
 numbers.  Instead, convert to a floating point number using the