Add information about passing procedures as parameters

[NHDaly]

I couldn't find this when i first read the documentation, and I came away assuming that Odin didn't support passing (pointers to) functions like you could do in C.

Of course, once I got more familiar I realized that procs are just values so they can be passed like any ordinary value. I wanted to spare a future reader that confusion, by making this more explicit.

Please feel free to edit/adjust any of the language here; it's my first time contributing to the Odin project. Thank you!