Improvements to PretrainedConfig documentation (#5642)

* Update PretrainedConfig doc

* Formatting

* Small fixes

* Forgotten args and more cleanup

docs/source/main_classes/configuration.rst

 Configuration
 ----------------------------------------------------
 
-The base class ``PretrainedConfig`` implements the common methods for loading/saving a configuration either from a local file or directory, or from a pretrained model configuration provided by the library (downloaded from HuggingFace's AWS S3 repository).
+The base class ``PretrainedConfig`` implements the common methods for loading/saving a configuration either from a
+local file or directory, or from a pretrained model configuration provided by the library (downloaded from
+HuggingFace's AWS S3 repository).
 
 ``PretrainedConfig``
 ~~~~~~~~~~~~~~~~~~~~~