Add versionadded marker for ctypes.c_bool.

Doc/lib/libctypes.tex

@@ -2298,6 +2298,7 @@ an integer address, or a string.
 Represent the C \code{bool} datatype (more accurately, _Bool from C99).
 Its value can be True or False, and the constructor accepts any object that
 has a truth value.
+\versionadded{2.6}
 \end{classdesc*}
 
 \begin{classdesc*}{HRESULT}