DOC: config: explain how default matching method for ACL works

DOC: config: explain how default matching method for ACL works

In issue #698, it's made apparent that the default matching method for
ACL keywords can be confusing when a converter is applied, because
depending on the converters used, users may think that the default
matching method from the sample fetch name might apply to the whole
expression. It's easier to understand that this doesn't make sense
when thinking about converters turning to completely different types
(e.g. hdr_beg(host),do_resolve() returns an IP, thus it's obvious
that _beg makes no sense at all).  This patch states this in the
doc to avoid future confusion.