Add link to Context LookupFlag discussion

https://github.com/harfbuzz/harfbuzz/discussions/3538

src/hb-ot-layout-gsubgpos.hh

@@ -1457,11 +1457,10 @@ static inline void apply_lookup (hb_ot_apply_context_t *c,
      *     NOT the one after it.
      *
      *   - If buffer length was decreased by n, it does not necessarily
-     *     mean that n match positions where removed, as there might
-     *     have been marks and default-ignorables in the sequence.  We
-     *     should instead drop match positions between current-position
-     *     and current-position + n instead. Though, am not sure which
-     *     one is better. Both cases have valid uses. Sigh.
+     *     mean that n match positions where removed, as there recursed-to
+     *     lookup might had a different LookupFlag.  Here's a constructed
+     *     case of that:
+     *     https://github.com/harfbuzz/harfbuzz/discussions/3538
      *
      * It should be possible to construct tests for both of these cases.
      */