Add note about ChunkSize explicit constructor since 4.4

[tcclevenger]

I chose to use a "note", but I'm happy to switch to a "warning" or bold text (done other places) or other suggestions.

[nliber]

I think that is fine. While the code fix is a breaking change, my expectation is that virtually no one took advantage of the previous behavior and if they did, the change to their code is minimal and not unexpected when working with C++.

[dalg24]

We don't want the explicit specifier here https://github.com/kokkos/kokkos-core-wiki/pull/561/files#diff-1e20fae80dae2837b5fdb19d79a210e4c1602b9e060f53f90f315eaa794a18d1R98
?

edit I mean besides the note you added

[tcclevenger]

@dalg24 added explicit specifier.