DOC: config: clarify the basics of ACLs (call point, multi-valued etc)

DOC: config: clarify the basics of ACLs (call point, multi-valued etc)

This is essentially in order to address the concerns expressed in
issue #2226 where it is mentioned that the moment they are called is
not clear enough. Admittedly, re-reading the paragraph doesn't make
it obvious on a quick read that they behave like functions. This patch
adds an extra paragraph that makes the parallel with programming
languages' boolean functions and explains the fact that they can be
multi-valued. Hoping this is clearer now.