add comment about the use of linear-gradient

Otherwise one of us is likely to return to the code in a few years,
think it could be made simpler, and then break things again. :-)

ts/editor/fields.scss

@@ -19,6 +19,8 @@
     background: var(--frame-bg);
 
     &.dupe {
+        // this works around the background colour persisting in copy+paste
+        // (https://github.com/ankitects/anki/pull/1278)
         background-image: linear-gradient(var(--flag1-bg), var(--flag1-bg));
     }
 }