Document behavior of the newly added System.Text.Json snake case and kebab case policies

[eiriktsarpalis]

.NET 8 includes built-in support for snake_case and kebab-case naming policies. Even though we recently updated the implementation to more closely match the semantics of the equivalent Json.NET feature, we intentionally deviated in certain cases where consensus is that the Json.NET implementation is flawed.

This divergence still has the potential of breaking users that are migrating from Json.NET to System.Text.Json, so we should try to document it. A good starting point is this commit whose test diffs highlight where exactly different outputs should be expected.

In addition to the above, we should consider documenting the fact that the naming policies (both old and new) don't support letters that are surrogate pairs. See dotnet/runtime#90352 for more details.

cc @gewarren @IEvangelist

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Associated WorkItem - 156741