Document that only the base queryset’s prefetch_related() is preserved when using union()

[Clifford Gama]

When using QuerySet.union() (possibly also intersection() and difference()) in Django, any prefetch_related() applied to the second (and third, etc) queryset is silently ignored. See ​forum thread in which it was suggested to open a documentation ticket. A docs improvement should also likely include the workaround for using Q objects to do all prefetches in the base qs (where possible), which is suggested by Simon in the forum.

[Natalia Bidart]

Makes sense, thank you Clifford!

[Chris M]

I have made an attempt at documenting this behavior in this ​PR.

I'm open to thoughts on how to improve this documentation: if it should exist somewhere else or if any changes are needed. It is a pretty nuanced topic, and I was trying to balance covering the details without making it too confusing.

I included an example of using Q objects to filter within the first prefetch_related. However, I'm not sure this is all that helpful of an example to provide. It feels like it introduces a bit too much complexity to the documentation.