doc: align repformer channel description with MP equation

Authored by OpenClaw (model: gpt-5.3-codex)

Author: njzjz-bot

doc/model/dpa2.md

@@ -27,7 +27,7 @@ The descriptor follows two main stages:
 1. **repinit (representation initializer)**
    - Initializes and fuses type and geometry information from local environments.
 1. **repformer (representation transformer)**
-   - Stacked message-passing layers that update $\mathbf{f}$ and $\mathbf{g}$ channels through convolution/symmetrization/MLP and attention-style interactions.
+   - Stacked message-passing layers that update $\mathbf{f}$, $\mathbf{g}$, and per-atom representations $\mathbf{h}$ through convolution/symmetrization/MLP and attention-style interactions.
 
 The final descriptor is formed from learned single-atom representations and then passed to downstream fitting/model components.