Add a comment to the top of the file explaining why the file is here.

Doc/lib/mimelib.tex

@@ -1,3 +1,10 @@
+% This document is largely a stub used to allow the email package docs
+% to be formatted separately from the rest of the Python
+% documentation.  This allows the documentation to be released
+% independently of the rest of Python since the email package is being
+% maintained for multiple Python versions, and on an accelerated
+% schedule.
+
 \documentclass{howto}
 
 \title{email Package Reference}