[Feedback]: Document LDAP filters

### Page URL

https://documentation.ubuntu.com/server/how-to/openldap/

### Select an option

- [ ] I found what I was looking for
- [x] I couldn't find what I was looking for
- [ ] I found the information, but it was incorrect
- [ ] I found the information, but it was incomplete
- [ ] I found the information, but it was confusing
- [ ] I encountered a technical issue (e.g., broken link, image not loading)

### Issue Description

LDAP filters are mentioned in a few places throughout the openldap documentation, but are not really explained in detail. Like ACLs, for example, they should have their own explanation page.

Start with the OpenLDAP documentation page at https://documentation.ubuntu.com/server/how-to/openldap/ and review its existing LDAP filter references alongside the ACL coverage. Identify the appropriate documentation location for a standalone explanation page. Done means LDAP filters are explained in their own page and the relevant references are covered.

Written by the indexing model from the issue text.