Add note of caution in `minimum_spanning_tree(_parallel)`

[czgdp1807]

Description of the problem

The concept of MST is valid only for connected and undirected graphs. If the graph is disconnected and undirected, the result will not be an MST but a forest consisting of many MSTs. However, for directed graphs, result can be anything.
To fix this issue just apply
pds_diff.txt
by using git apply [path to pdf_diff.txt].

Example of the problem

References/Other comments

.. [1] https://en.wikipedia.org/wiki/Edmonds%27_algorithm

[sar99]

Hello sir,
Can I submit a PR for this issue?

[czgdp1807]

Hi @sar99
Sure. Are you participating through gssoc20?

[sar99]

Yes sir, I am a participant in gssoc20.