Add links to issues discussing the change in Release Notes

[kuchta]

Guys, many thanks for the new RC: https://nim-lang.org/blog/2022/12/21/version-20-rc.html

There are a lot of great changes, but unfortunatelly there are some I don't understand the reasoning for like:

Previously, calls like foo(a, b): ... or foo(a, b) do: ... where the final argument of foo had type proc () were assumed by the compiler to mean foo(a, b, proc () = ...). This behavior is now deprecated. Use foo(a, b) do (): ... or foo(a, b, proc () = ...) instead.

So it would be immensely valuable to link each change with some issue(s) discussing that feature (removal) to have some background.

Anyway, great work, thank you very much ❤️