Specify how function pointers will handle context registers

[kotlarmilos]

Description

This PR adds more information on how function pointers will handle context registers.

[jkotas]

The proposal mentions CallConvSwift in a few places, but it does not say what it is.

Would it make sense to explicitly mention CallConvSwift at the end of the initial paragraph on calling convention and say that it can be used in all positions where other existing calling convention modifiers can be used, that means: UnmanagedCallConvAttribute for P/Invoke, UnmanagedCallersOnlyAttribute.CallConv for reverse P/Invoke and unmanaged function pointers (delegate* unmanaged[Swift]`).

[AaronRobinsonMSFT]

This LGTM.

We should make sure @jkoritzinsky signs off on this from the CoreCLR side too.