w6c: unstale the cg_sret_retsize header doc for the #38 tagged arm (#38 review)

cmd/w6c/cgen.c

@@ -313,8 +313,10 @@ fld_isfloat(Type *t, int *isf32)
  *     via sret instead of registers (#10). The element footprint walk
  *     matches the N_RETURN tuple SEND exactly (a float = 1 SSE
  *     eightbyte, a slice/str its 3-word header, a scalar 1 GP word).
- * Everything else (in-cap tuples, tagged unions, str, slices, scalars)
- * routes through its register-return ABI → 0. */
+ *   - TY_TAGGED whose slot exceeds the AX/DX/CX/R8 cursor
+ *     (> TUPLE_GPCAP eightbytes) → its natural size (#38).
+ * Everything else (in-cap tuples, in-cap tagged unions, str, slices,
+ * scalars) routes through its register-return ABI → 0. */
 static int
 cg_sret_retsize(Type *rt)
 {