Add info about wildcard functionality for core override settings

[Terry Janas]

It should be noted in the documentation that the corename in MiSTer.ini can support wildcards. Example:
[dkong*] ; for all cores with a CORENAME that begins with dkong

Also, [arcade] can be used to provide default settings for all arcade cores.

[Kevin Coleman]

Thanks! Great point!

[Terry Janas]

Fixed via 9cee762