Where is the documentation for the ACL module/api?
If you take a look at the ACL project page, you'll see that the following four client modules exist:


Content Access
Flexi Access 
Forum Access
Image Gallery Access


You'll find plenty of examples on how to use the API in the source code of those.

The documentation for ACL are the comments embedded inside acl.module.  They are quite helpful when you try to create a client module.

P.S.: I am the maintainer of Flexi Access.  I can verify that ACL works as advertised.