docs: warn that fetchone() materializes the entire result set

dqlite returns all matching rows in one response (no server-side
cursors). fetchone() delegates to fetch() which reads all rows into
memory. Add a docstring note advising users to add LIMIT 1 for large
result sets.

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

Author: antoineleclair

src/dqliteclient/connection.py

@@ -274,7 +274,12 @@ async def fetchall(self, sql: str, params: Sequence[Any] | None = None) -> list[
     async def fetchone(
         self, sql: str, params: Sequence[Any] | None = None
     ) -> dict[str, Any] | None:
-        """Execute a query and return the first result."""
+        """Execute a query and return the first result.
+
+        Note: dqlite returns all matching rows over the wire. For large
+        result sets, add ``LIMIT 1`` to your query to avoid excessive
+        memory usage.
+        """
         results = await self.fetch(sql, params)
         return results[0] if results else None