Revise "decimal' docs, adding note about flag.

Author: nascheme

Doc/library/decimal.rst

@@ -1884,13 +1884,20 @@ the current thread.
 
 If :func:`setcontext` has not been called before :func:`getcontext`, then
 :func:`getcontext` will automatically create a new context for use in the
-current thread.
-
-The new context is copied from a prototype context called *DefaultContext*. To
-control the defaults so that each thread will use the same values throughout the
-application, directly modify the *DefaultContext* object. This should be done
-*before* any threads are started so that there won't be a race condition between
-threads calling :func:`getcontext`. For example::
+current thread.  New context objects have default values set from the
+:data:`decimal.DefaultContext` object.
+
+The :data:`sys.flags.thread_inherit_context` flag affects the context for
+new threads.  If the flag is false, new threads will start with an empty
+context.  In this case, :func:`getcontext` will create a new context object
+when called and use the default values from *DefaultContext*.  If the flag
+is true, new threads will start with a copy of context from the caller of
+:meth:`Thread.start`.
+
+To control the defaults so that each thread will use the same values throughout
+the application, directly modify the *DefaultContext* object. This should be
+done *before* any threads are started so that there won't be a race condition
+between threads calling :func:`getcontext`. For example::
 
    # Set applicationwide defaults for all threads about to be launched
    DefaultContext.prec = 12