doc: Clarify is_decoder usage in PretrainedConfig documentation (#36724)

* fix: clarify decoder usage in PretrainedConfig documentation

* Apply suggestions from code review

updated doc

Co-authored-by: Steven Liu <59462357+stevhliu@users.noreply.github.com>

---------

Co-authored-by: Steven Liu <59462357+stevhliu@users.noreply.github.com>

Author: d-kleine

src/transformers/configuration_utils.py

@@ -109,7 +109,7 @@ class PretrainedConfig(PushToHubMixin):
         is_encoder_decoder (`bool`, *optional*, defaults to `False`):
             Whether the model is used as an encoder/decoder or not.
         is_decoder (`bool`, *optional*, defaults to `False`):
-            Whether the model is used as decoder or not (in which case it's used as an encoder).
+            Whether to only use the decoder in an encoder-decoder architecture, otherwise it has no effect on decoder-only or encoder-only architectures.
         cross_attention_hidden_size** (`bool`, *optional*):
             The hidden size of the cross-attention layer in case the model is used as a decoder in an encoder-decoder
             setting and the cross-attention hidden dimension differs from `self.config.hidden_size`.