[Exim] Doc suggestion

It seems there is no general description for the various 'Type:' of
config options in the spec, at least not that I can find. I was
specifically looking to see what I could put for mx_domains, if
'domain.com' was sufficient, or if I needed '*.domain.com'. Of course,
its possible I know already, and since its 5:30 PM I just can't recall
;)

Anyway, perhaps there should be a formal definition of what a
'host-list' or a 'domain-list', or any of the other types of options...

Long-time exim users may know all these - but shouldnt it be there for
the uninitiated?

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