add more docs

docs/ORTModule_Training_Guidelines.md

@@ -307,7 +307,7 @@ A classical usage of disabling the deep copy: when the deep copy before module e
 #### ORTMODULE_ATEN_SDPA_FALLBACK
 
 - **Feature Area**: *ORTMODULE/Optimizations*
-- **Description**: By default, this is disabled. This env var can be used for enabling pre-export attention fall back to PyTorch's efficient_attention ATen kernel for execution.
+- **Description**: By default, this is disabled. This env var can be used for enabling pre-export attention fall back to PyTorch's efficient_attention ATen kernel for execution. NOTE: will not work if model uses both masked and unmasked attention, can only be one.
 
     ```bash
     export ORTMODULE_ATEN_SDPA_FALLBACK=1 # ENABLE **WITHOUT** ATTN_MASK INPUT