src/sage/tests/books/comp.../integration_doctest.py: update header comment

It's no longer safe to blindly overwrite this file, since we have
added a "# long time" annotation.

Author: orlitzky

src/sage/tests/books/computational-mathematics-with-sagemath/integration_doctest.py

@@ -1,11 +1,8 @@
 """
-This file (./integration_doctest.sage) was *autogenerated* from ./integration.tex,
-with sagetex.sty version 2011/05/27 v2.3.1.
-It contains the contents of all the sageexample environments from this file.
-You should be able to doctest this file with:
-sage -t ./integration_doctest.sage
-It is always safe to delete this file; it is not used in typesetting your
-document.
+This file was automatically generated from ./integration.tex (in
+the book) and included into the SageMath source tree. It may have been
+modified slightly; if you re-generate it, please diff the changes to
+prevent any important modifications from being overwritten.
 
 Sage example in ./integration.tex, line 73::