Specify that nested *MDRange policies are limited to a sum reduction

[pzehner]

Following a discussion on Slack, TeamVectorMDRange, TeamThreadMDRange, and ThreadVectorMDRange limit the reduction to a sum. The documentation is updated in this way.

I am not sure where to update the text, however.

[masterleinad]

I would add a note for the parallel_reduce page and possibly for the Team*MDrange pages. The limitation doesn't really have anything to do with built-in reducers but generally that reducers cannot be used and we only support a scalar as return type.