Document objects that can be used with the `with` statement.

Doc/lib/libdecimal.tex

@@ -442,9 +442,33 @@ the \function{getcontext()} and \function{setcontext()} functions:
   Set the current context for the active thread to \var{c}.                                          
 \end{funcdesc}  
 
-New contexts can formed using the \class{Context} constructor described below.
-In addition, the module provides three pre-made contexts:                                          
+Beginning with Python 2.5, you can also use the \keyword{with} statement
+to temporarily change the active context. For example the following code
+increases the current decimal precision by 2 places, performs a
+calculation, and then automatically restores the previous context:
 
+\begin{verbatim}
+from __future__ import with_statement
+import decimal
+
+with decimal.getcontext() as ctx:
+    ctx.prec += 2   # add 2 more digits of precision
+    calculate_something()
+\end{verbatim}
+
+The context that's active in the body of the \keyword{with} statement is
+a \emph{copy} of the context you provided to the \keyword{with}
+statement, so modifying its attributes doesn't affect anything except
+that temporary copy.
+
+You can use any decimal context in a \keyword{with} statement, but if
+you just want to make a temporary change to some aspect of the current
+context, it's easiest to just use \function{getcontext()} as shown
+above.
+
+New contexts can also be created using the \class{Context} constructor
+described below. In addition, the module provides three pre-made
+contexts:
 
 \begin{classdesc*}{BasicContext}
   This is a standard context defined by the General Decimal Arithmetic