docs: fix misleading close() docstring — only idle connections are closed

The close() docstring said "Close all connections (both idle and
in-use)" but since the #080 fix, it only closes idle connections.
In-use connections are closed when their acquire() context manager
exits. Update the docstring to accurately describe this behavior
and guide users on how to ensure full cleanup.

Closes #092

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: antoineleclair

src/dqliteclient/pool.py

@@ -185,7 +185,14 @@ async def fetch(self, sql: str, params: Sequence[Any] | None = None) -> list[dic
             return await conn.fetch(sql, params)
 
     async def close(self) -> None:
-        """Close all connections (both idle and in-use)."""
+        """Close the pool and all idle connections.
+
+        Sets the pool as closed so no new connections can be acquired.
+        Idle connections are closed immediately. In-use connections are
+        closed when they are returned (when the acquire() context manager
+        exits). To ensure all connections are closed, cancel or await
+        in-flight tasks before calling close().
+        """
         self._closed = True
 
         # Close idle connections