acl: add docs and adapt visibility

acl: add docs and adapt visibility

document all public things, add some doc links and make some
previously-public things only available for test cases or within the
crate:

previously public, now private:
- AclTreeNode::extract_user_roles (we have extract_roles())
- AclTreeNode::extract_group_roles (same)
- AclTreeNode::delete_group_role (exists on AclTree)
- AclTreeNode::delete_user_role (same)
- AclTreeNode::insert_group_role (same)
- AclTreeNode::insert_user_role (same)
- AclTree::write_config (we have save_config())
- AclTree::load (we have config()/cached_config())

previously public, now crate-internal:
- AclTree::from_raw (only used by tests)
- split_acl_path (used by some test binaries)

Signed-off-by: Fabian Grünbichler <f.gruenbichler@proxmox.com>