Clarify that include files may not contain job sections

Signed-off-by: Jens Axboe <axboe@fb.com>
diff --git a/HOWTO b/HOWTO
index 887c276..aaa46f8 100644
--- a/HOWTO
+++ b/HOWTO
@@ -190,7 +190,8 @@
 
 Settings pulled into a section apply to that section only (except global
 section). Include directives may be nested in that any included file may
-contain further include directive(s).
+contain further include directive(s). Include files may not contain []
+sections.
 
 
 4.1 Environment variables