new section - builtin constants

Doc/lib/libconsts.tex

+\section{Builtin Constants}
+
+A small number of constants live in the builtin namespace.  They are:
+
+\begin{datadesc}{False}
+The false value of the \code{bool} type.
+\versionadded{2.3}
+\end{datadesc}
+
+\begin{datadesc}{True}
+The true value of the \code{bool} type.
+\versionadded{2.3}
+\end{datadesc}
+
+\begin{datadesc}{None}
+The sole value of \code{NoneType}.  \code{None} is frequently used to
+represent the absence of a value, as when default arguments are not passed
+to a function.
+\end{datadesc}
+