Document when you should and when you don't have to ship a task `.deps.json`

[rainersigwald]

See discussion in https://github.com/dotnet/roslyn/issues/74725, but:

• If a task assembly has an adjacent .deps.json, MSBuild will respect it.
• Generally you should probably ship this
• But you don't super have to, the old "load references from next to it" behavior covers a lot.

We should write down details.