docs: add __aenter__/__aexit__ one-liner docstrings for twin parity with redis

Signed-off-by: disqualifier <dev@disqualifier.me>

src/mysql/mysql.py

@@ -150,9 +150,11 @@ class MysqlDB:
             self._pool = None
 
     async def __aenter__(self) -> "MysqlDB":
+        """enter: connect() and return self"""
         return await self.connect()
 
     async def __aexit__(self, exc_type, exc, tb) -> None:
+        """exit: close(), ignoring exc_type/exc/tb"""
         await self.close()
 
     @property