[Doc] Sparse adstack heap: prefix qd.field compound-index gate limitation bullet with ⚠️ emoji marker

Author: duburcqa

docs/source/user_guide/autodiff.md

@@ -354,7 +354,7 @@ A large `ndrange` combined with several loop-carried variables multiplies quickl
   - pass `ad_stack_size=N` to `qd.init()` with `N` large enough to cover the real push count (bypasses the sizer).
 - **Out-of-memory before the kernel even runs.** A reverse pass through many loop-carried variables at a large ndrange can ask the runtime for more adstack memory than the device can physically back, even when the sizer's number is correct. Surfaces as an allocator OOM at launch time. Remedies are the ones listed under *Avoiding OOM on GPU* above: fewer loop-carried variables, a smaller ndrange, manual checkpointing, or more device-memory headroom.
 - **Loop bounds backed by a mutated ndarray.** A reverse-mode kernel with `for i in range(n[j])` requires `n[j]` to hold the same value at the forward call and at `.grad()`. If anything writes to `n[j]` between those two points - the differentiable kernel itself, or any other kernel call - the computed gradient may come out wrong, sometimes as an `Adstack overflow` exception at `qd.sync()`, sometimes silently. The safe rule: populate loop-bound ndarrays before the forward call and leave them untouched until `.grad()` returns. The reason for that is Quadrants' adstack sizer design: it reads the loop bound separately at each dispatch, which includes forward and backward calls. Tape-based eager AD like [PyTorch's autograd](https://pytorch.org/docs/stable/notes/autograd.html) is not affected, since the trip count is recorded as the forward runs and reused at backward time.
-- **Gate on a `qd.field` indexed by an expression that is not a plain loop variable.** A reverse-mode kernel of the shape `for i in range(n): if field[i % K] > eps: <adstack work>` (or any gate whose index is not a plain loop variable - `field[2 * i]`, `field[42]`, `field[other_field[i]]`) may produce silently wrong gradients. Workarounds:
+- :warning: **Gate on a `qd.field` indexed by an expression that is not a plain loop variable.** A reverse-mode kernel of the shape `for i in range(n): if field[i % K] > eps: <adstack work>` (or any gate whose index is not a plain loop variable - `field[2 * i]`, `field[42]`, `field[other_field[i]]`) may produce silently wrong gradients. Workarounds:
   - raise `ad_stack_sparse_threshold_bytes` in `qd.init()` past the kernel's conservative-heap byte size;
   - use a `qd.ndarray` for the gating field instead of a `qd.field`.