Docs on how to output the process tree

[guettli]

I searched the docs for "tree", and found no result.

Please up date the docs and explain how to traverse the process tree.

Example: how to output something like "ps aux --forest" does?

[giampaolo]

I provided an example script: https://github.com/giampaolo/psutil/blob/master/examples/pstree.py. I don't think this is a task which is common enough so that it should be mentioned in the doc.