MDL-55388 theme_noname: Define the purpose of bootstrap-rtl.scss

Add a comment explaining what the bootstrap-rtl.scss file is for (and what it is not for).

Part of MDL-55071

theme/noname/scss/moodle/bootstrap-rtl.scss

@@ -1,7 +1,14 @@
+/**
+ * Bootstrap overrides for RTL
+ *
+ * This file is only for overriding sass from upstream bootstrap, all general rtl fixes for
+ * moodle scss should be placed immediately after the definition of the ltr rule.
+ */
+
 .breadcrumb-item {
     + .breadcrumb-item::before {
         content: "#{$breadcrumb-divider-rtl}";
         /*rtl:remove*/
         content: "#{$breadcrumb-divider}";
     }
-}
+}