Adjust *is_causal documentation

According to pytorch#97214, as discussed in pytorch#97166.

torch/nn/modules/transformer.py

@@ -203,9 +203,13 @@ def forward(
         Args:
             src: the sequence to the encoder (required).
             mask: the mask for the src sequence (optional).
-            is_causal: If specified, applies a causal mask as mask (optional)
-                and ignores attn_mask for computing scaled dot product attention.
+            is_causal: If specified, applies a causal mask as mask.
                 Default: ``False``.
+                Warning:
+                ``is_causal`` provides a hint that ``mask`` is the
+                causal mask. Providing incorrect hints can result in
+                incorrect execution, including forward and backward
+                compatibility.
             src_key_padding_mask: the mask for the src keys per batch (optional).
 
         Shape:
@@ -361,9 +365,20 @@ def forward(self, tgt: Tensor, memory: Tensor, tgt_mask: Optional[Tensor] = None
             tgt_key_padding_mask: the mask for the tgt keys per batch (optional).
             memory_key_padding_mask: the mask for the memory keys per batch (optional).
             tgt_is_causal: If specified, applies a causal mask as tgt mask.
-                Mutually exclusive with providing tgt_mask. Default: ``False``.
-            memory_is_causal: If specified, applies a causal mask as memory mask.
-                Mutually exclusive with providing memory_mask. Default: ``False``.
+                Default: ``False``.
+                Warning:
+                ``tgt_is_causal`` provides a hint that ``tgt_mask`` is
+                the causal mask. Providing incorrect hints can result in
+                incorrect execution, including forward and backward
+                compatibility.
+            memory_is_causal: If specified, applies a causal mask as
+                memory mask.
+                Default: ``False``.
+                Warning:
+                ``memory_is_causal`` provides a hint that
+                ``memory_mask`` is the causal mask. Providing incorrect
+                hints can result in incorrect execution, including
+                forward and backward compatibility.
 
         Shape:
             see the docs in Transformer class.
@@ -493,8 +508,13 @@ def forward(
         Args:
             src: the sequence to the encoder layer (required).
             src_mask: the mask for the src sequence (optional).
-            is_causal: If specified, applies a causal mask as src_mask.
-              Default: ``False``.
+            is_causal: If specified, applies a causal mask as src mask.
+                Default: ``False``.
+                Warning:
+                ``is_causal`` provides a hint that ``src_mask`` is the
+                causal mask. Providing incorrect hints can result in
+                incorrect execution, including forward and backward
+                compatibility.
             src_key_padding_mask: the mask for the src keys per batch (optional).
 
         Shape:
@@ -706,9 +726,20 @@ def forward(
             tgt_key_padding_mask: the mask for the tgt keys per batch (optional).
             memory_key_padding_mask: the mask for the memory keys per batch (optional).
             tgt_is_causal: If specified, applies a causal mask as tgt mask.
-                Mutually exclusive with providing tgt_mask. Default: ``False``.
-            memory_is_causal: If specified, applies a causal mask as memory mask.
-                Mutually exclusive with providing memory_mask. Default: ``False``.
+                Default: ``False``.
+                Warning:
+                ``tgt_is_causal`` provides a hint that ``tgt_mask`` is
+                the causal mask. Providing incorrect hints can result in
+                incorrect execution, including forward and backward
+                compatibility.
+            memory_is_causal: If specified, applies a causal mask as
+                memory mask.
+                Default: ``False``.
+                Warning:
+                ``memory_is_causal`` provides a hint that
+                ``memory_mask`` is the causal mask. Providing incorrect
+                hints can result in incorrect execution, including
+                forward and backward compatibility.
         Shape:
             see the docs in Transformer class.
         """