Intersect by and union documentation in base

[Bram Neijt]

Dear library maintainers,

I've used both intersectBy and unionBy from the Base.List module.

For intersectBy the documentation did not mention which element will
be used when an element is in both lists.

The attached patch adds a line to the documentation of intersectBy,
where I think it makes sense to add this information.

The unionBy documentaiton has the same kind of problem, however I did
not patch that because I'm not sure whether this is part of the
contract union whats to take on (I mean being deterministic in which
list is the preferred source of elements).

Please consider updating the documentation and feel free to contact me
for more information if needed.

Greetings,

Bram
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