Update documentation for `System.Numerics.Vector<T>` to be more explicit.

[tannergooding]

This was raised in the dotnet/corefx gitter channel.

It is not currently clear from the documentation that the types exposed by the System.Numerics.Vectors assembly are not meant for general purpose Vector operations and that the types are heavily tied to the underlying hardware.

The documentation for this could be greatly improved.

Some types, such as Vector<T> should have explicit (and readily visible) documentation that its element count is limited to Vector<T>.Count, and that certain methods (such as the Vector(T[]) constructor) will only use the first Vector<T>.Count elements and ignore the remaining.

[aliostad]

Thank you!

[danmoseley]

I think this issue probably should be in https://github.com/dotnet/docs.

[karelz]

We need to propose wording changes in docs, then move it to docs repo.
Anyone interested?

[Gnbrkm41]

Opened a fix PR that (hopefully) points these out in dotnet/dotnet-api-docs repo.