Mention if duplicates are allowed in maps in the CDDL files

In `conway.cddl`, there are many types which are maps.
As per CDDL spec, maps are allowed to have duplicate keys.
But the node doesn't allow duplicate keys in some places, and allows in some other places.

For example, the multiasset type is defined as a map:
https://github.com/IntersectMBO/cardano-ledger/blob/28ab3884cac8edbb7270fd4b8628a16429d2ec9e/eras/conway/impl/cddl-files/conway.cddl#L564

The mint type, which is an instance of multiasset, can have duplicate keys.
https://github.com/IntersectMBO/cardano-ledger/blob/28ab3884cac8edbb7270fd4b8628a16429d2ec9e/eras/conway/impl/cddl-files/conway.cddl#L576

But the multiasset field of the value type can't have.
https://github.com/IntersectMBO/cardano-ledger/blob/28ab3884cac8edbb7270fd4b8628a16429d2ec9e/eras/conway/impl/cddl-files/conway.cddl#L574

It would be helpful for library/tools authors to have comments in the CDDL file around map types, describing whether duplicate keys are allowed or not.