docs: add dflash section

Author: kashif

docs/speculative.md

@@ -52,6 +52,32 @@ Supported EAGLE-3 draft models include:
 
 For the full and up-to-date list of supported models, see #18039.
 
+### DFlash (`draft-dflash`)
+
+DFlash produces an entire block of draft tokens in a single forward pass (block diffusion) and
+injects the target model's hidden states into the draft model's attention, instead of drafting one
+token at a time. This keeps the draft model small while making drafting GPU-friendly. Unlike EAGLE-3
+(a single-layer autoregressive draft), the DFlash draft uses several transformer layers but emits a
+whole block per draft step.
+
+The draft is a small block-diffusion model trained for a specific target (for example
+`z-lab/Qwen3-4B-DFlash` for `Qwen/Qwen3-4B`). Convert it with `--target-model-dir` so it inherits the
+target's tokenizer and token embeddings:
+
+```bash
+python convert_hf_to_gguf.py z-lab/Qwen3-4B-DFlash \
+    --target-model-dir Qwen/Qwen3-4B --outtype bf16 --outfile Qwen3-4B-DFlash.gguf
+
+llama-server -m Qwen3-4B.gguf -md Qwen3-4B-DFlash.gguf \
+    --spec-type draft-dflash --spec-draft-n-max 15 -fa on --jinja
+```
+
+`--spec-draft-n-max` is clamped to the draft model's trained block size.
+
+See:
+
+- #22105
+
 ### n-gram Cache (`ngram-cache`)
 
 An n-gram is a sequence of n tokens. The n-gram cache implementation maintains statistics about short n-gram sequences.