MDL-58725 course: include selector change notes in format/upgrade.txt

course/format/upgrade.txt

@@ -14,7 +14,10 @@ Overview of this plugin type at http://docs.moodle.org/dev/Course_formats
   in the course_sections table is treated as a number of sections (excluding section 0 that should always be present).
 * Method create_course() will populate the new course with empty sections if $data->numsections is provided even if
   "numsections" is not an option defined by the course format.
-* course/changenumsections.php can now be used to insert sections at any positions
+* course/changenumsections.php can now be used to insert sections at any positions.
+* The variable SECTIONLEFTSIDE was changed from '.left .section-handle img' to '.left .section-handle .icon' in
+  course/format/topics/format.js and course/format/weeks/format.js due to the introduction of Font Awesome (see MDL-40759)
+  which altered the HTML. Please check if your course format relies on this selector, and if so, update it.
 
 === 3.2 ===
 * Callback delete_course is deprecated and should be replaced with observer for event \core\event\course_content_deleted