Update structure marshaling guidance for .NET Core 3.0

[nxtn]

Contributes to #11396

Continued from #12101

dotnet/coreclr#22041 fixed marshaling explicit-layout structures on non-Windows platforms.

/cc @jkoritzinsky

[nxtn]

@mairaw

@jkoritzinsky is not identified as the code owner. #15355

[Youssef1313]

@mairaw

@jkoritzinsky is not identified as the code owner. #15355

https://help.github.com/en/github/creating-cloning-and-archiving-repositories/about-code-owners

The people you choose as code owners must have write permissions for the repository.

I'm not sure if he has write permissions in dotnet/docs repo, but if he doesn't have write permissions then that is the problem.

[nxtn]

I'm not sure if he has write permissions

The approval marks of those who have write access are green.

[Youssef1313]

I'm not sure if he has write permissions

The approval marks of those who have write access are green.

Well, then this is the problem.

[mairaw]

We're revisiting all the assignments, code owners, perms, etc. I'm still in the process of doing this after Ron left but I can review and approve this in the meantime given that @jkoritzinsky already approved this.

[mairaw]

Small request before we merge

[mairaw]

Thanks for making the additional change.