Add an information about the necessary constraint type for the linked collections

[natfirst]

Add the information right after the description of constraint types. Users should understand how to check associated entities. https://doc.cuba-platform.com/manual-latest/constraints.html See: https://www.cuba-platform.com/discuss/t/direct-m2m-with-access-group-constraint/5902/2

[jreznot]

I think we should emphasize that:

• Database constraints are not applied for reference collections.
• In-memory constrains are applied for them.