Sankey hover info

I am trying to change the hoverinfo of sankey diagrams with hoverinfo. I want to change the default “source” and “target”. However, following info on hoverinfo isn’t very helpful to me:

What does “Any combination of [] joined with ‘+’ characters” mean?
“[x]+[y]” or
[“x”]+[“y”] or
‘[“x”]+[“y”]’ or
[x]+[y] or
“x”+“y” or

Well I tried them all, none worked.

Invalid value of type ‘builtins.str’ received for the ‘hoverinfo’ property of sankey
Received value: ‘Sourc+Targ’

The 'hoverinfo' property is a flaglist and may be specified
as a string containing:
  - Any combination of [] joined with '+' characters
    (e.g. '')
    OR exactly one of ['all', 'none', 'skip'] (e.g. 'skip')

It means “x+y”