Clarify documentation of get_shortest_path

The documentation for [`get_shortest_path`](https://python.igraph.org/en/stable/api/igraph.GraphBase.html#get_shortest_path) should mention that it returns only a single path, and that one should use [`get_all_shortest_paths`](https://python.igraph.org/en/stable/api/igraph.GraphBase.html#get_all_shortest_paths) to get all shortest paths.

**Version information**
Version 0.11.3
