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.

doc/configuration.txt

@@ -19177,10 +19177,19 @@ data called patterns.
 7.1. ACL basics
 ---------------
 
-The use of Access Control Lists (ACL) provides a flexible solution to perform
-content switching and generally to take decisions based on content extracted
-from the request, the response or any environmental status. The principle is
-simple :
+Access Control Lists (ACL) consist in declaring a named method to compare any
+piece of information against a list of pre-defined patterns. They should be
+seen as practically equivalent to functions in most programming languages, in
+that their declaration makes them available to be later called when needed.
+Their evaluation only returns a match or a mismatch, which is comparable to
+booleans in many programming languages. Contrary to functions in programming
+languages, ACLs may be overloaded as many times as needed in order to define
+additional matching methods for the same name. In this case they will all be
+evaluated in their declaration order until one matches.
+
+The use of ACLs provides a flexible solution to perform content switching and
+generally to take decisions based on content extracted from the request, the
+response or any environmental status. The principle is simple :
 
   - extract a data sample from a stream, table or the environment
   - optionally apply some format conversion to the extracted sample