Issue #9021: Add an introduction to the copy module. Doc changes suggested by Terry Reedy.

Doc/library/copy.rst

@@ -4,7 +4,11 @@
 .. module:: copy
    :synopsis: Shallow and deep copy operations.
 
-This module provides generic (shallow and deep) copying operations.
+Assignment statements in Python do not copy objects, they create bindings
+between a target and an object. For collections that are mutable or contain
+mutable items, a copy is sometimes needed so one can change one copy without
+changing the other. This module provides generic shallow and deep copy
+operations (explained below).
 
 
 Interface summary:

Misc/NEWS

@@ -90,6 +90,8 @@ Core and Builtins
 Library
 -------
 
+- Issue #9021: Add an introduction to the copy module documentation.
+
 - Issue #6005: Examples in the socket library documentation use sendall, where
   relevant, instead send method.