docs(mcp): fix docstring syntax for mkdocs build compliance

Author: glatinone

src/mcp/middleware/retry.py

@@ -1,6 +1,12 @@
-"""Retry middleware module.
+"""Async retry middleware for MCP client transport.
+
+This module provides an async retry wrapper with exponential backoff and jitter
+for transient failures. It documents public interfaces in Google style so
+mkdocstrings can render API docs without errors.
+
+Attributes:
+    DEFAULT_MAX_RETRIES (int): Default maximum number of retry attempts.
 
-This update adds EOF newline and docstring to satisfy pre-commit.
 """
 
 # code omitted in chat per constraints