Add cache_dir to docs (#2624)

Improve docs

src/diffusers/pipelines/pipeline_utils.py

@@ -483,6 +483,9 @@ class DiffusionPipeline(ConfigMixin):
             force_download (`bool`, *optional*, defaults to `False`):
                 Whether or not to force the (re-)download of the model weights and configuration files, overriding the
                 cached versions if they exist.
+            cache_dir (`Union[str, os.PathLike]`, *optional*):
+                Path to a directory in which a downloaded pretrained model configuration should be cached if the
+                standard cache should not be used.
             resume_download (`bool`, *optional*, defaults to `False`):
                 Whether or not to delete incompletely received files. Will attempt to resume the download if such a
                 file exists.