MDL-78806 core: Add upgrade.txt notes

lib/upgrade.txt

@@ -167,6 +167,11 @@ being forced open in all behat tests.
   the grade item once the recalculations are completed. (This was fixed in 4.3, 4.2.2)
 * Added a new constant called MAX_PASSWORD_CHARACTERS in moodlelib.php to hold a length of accepted password.
 * Added a new method called exceeds_password_length in moodlelib.php to validate the password length.
+* \moodle_page::set_title() has been updated to append the site name depending on the value of $CFG->sitenameintitle and whether
+  the site's fullname/shortname has been set. So there's no need to manually add the site name whenever calling $PAGE->set_title().
+  If it's necessary to override this, pass `false` to its new optional parameter `$appendsitename`.
+* New page title separator constant `moodle_page:TITLE_SEPARATOR` has been created to help standardise the separators used in page
+  titles.
 
 === 4.2 ===