Change all documentation examples using variable-length tuples into lists

[Adam Johnson]

In many places in the documentation we have examples using varible-length tuples, for example ModelAdmin.fields which takes any number of fields.

For all the reasons I wrote up in my post ​tuples versus lists I think it would be good to convert all such examples to lists. This would make the documentation more beginner-friendly and less prone to propagating these mistakes.

There's also a general guideline I saw (from Guido maybe?) that tuples are intended for fixed-length records where the position is part of the meaning - e.g. (x, y) coordinates - and lists are for variable-length collections of homogenous-meaning items. So all our variable-length tuples would be better as lists under this guideline.

[Mariusz Felisiak]

Closing as duplicate of #30947. I think it will be good to keep all related cleanups in a single ticket.

[Adam Johnson]

Thanks for knowing about/finding the dupe!