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

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

src/psql/psql.py

@@ -164,9 +164,11 @@ class PsqlDB:
             self._pool = None
 
     async def __aenter__(self) -> "PsqlDB":
+        """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