add docstrings to deprecated properties

Author: mattsb42-aws

src/aws_encryption_sdk/streaming_client.py

@@ -755,11 +755,13 @@ def _read_header(self):
 
     @property
     def body_start(self):
+        """Log deprecation warning when body_start is accessed."""
         _LOGGER.warning("StreamDecryptor.body_start is deprecated and will be removed in 1.4.0")
         return self._body_start
 
     @property
     def body_end(self):
+        """Log deprecation warning when body_end is accessed."""
         _LOGGER.warning("StreamDecryptor.body_end is deprecated and will be removed in 1.4.0")
         return self._body_end