Update ReflectionOnly methods for .NET Core

[sdmaclea]

Add System.PlatformNotSupportedException doc.

Summary

Update the ReflectionOnly methods for .NET Core. These methods are implemented in .NET Core as

throw new PlatformNotSupportedException(...);

Add missing

<exception cref="T:System.PlatformNotSupportedException">
    .NET Core only: This member is not supported.
</exception>

entries.

@mairaw @jkotas @vitek-karas

[rpetrusha]

Thanks for adding exception information for .NET Core, @sdmaclea. I'll merge your PR now.