Add version added info for with statement and context managers

Doc/lib/libcontextlib.tex

@@ -4,6 +4,8 @@
 \declaremodule{standard}{contextlib}
 \modulesynopsis{Utilities for \keyword{with}-statement contexts.}
 
+\versionadded{2.5}
+
 This module provides utilities for common tasks involving the
 \keyword{with} statement.
 

Doc/ref/ref3.tex

@@ -2109,6 +2109,8 @@ implement a \method{__coerce__()} method, for use by the built-in
 
 \subsection{Context Managers and Contexts\label{context-managers}}
 
+\versionadded{2.5}
+
 A \dfn{context manager} is an object that manages the entry to, and exit
 from, a \dfn{context} surrounding a block of code.  Context managers are
 normally invoked using the \keyword{with} statement (described in

Doc/ref/ref7.tex

@@ -312,6 +312,8 @@ statement to generate exceptions may be found in section~\ref{raise}.
 \section{The \keyword{with} statement\label{with}}
 \stindex{with}
 
+\versionadded{2.5}
+
 The \keyword{with} statement is used to wrap the execution of a block
 with methods defined by a context manager (see
 section~\ref{context-managers}). This allows common