bpo-36261: Improve example of the preamble field in email docs (GH-14751)

(cherry picked from commit 8efade91b12a13102a09a3856179021e579da5e9)

Co-authored-by: Carl Bordum Hansen <carl@bordum.dk>
diff --git a/Doc/includes/email-mime.py b/Doc/includes/email-mime.py
index c610242..6af2be0 100644
--- a/Doc/includes/email-mime.py
+++ b/Doc/includes/email-mime.py
@@ -14,7 +14,7 @@
 # family = the list of all recipients' email addresses
 msg['From'] = me
 msg['To'] = ', '.join(family)
-msg.preamble = 'Our family reunion'
+msg.preamble = 'You will not see this in a MIME-aware mail reader.\n'
 
 # Open the files in binary mode.  Use imghdr to figure out the
 # MIME subtype for each specific image.