incorrect docs for idlist-delimiter Drush option

Problem/Motivation

> #[CLI\Option(name: 'idlist-delimiter', description: 'The delimiter for records')]

This isn't right. The delimiter is for IDs within a record:

> --idlist=1:a,2:b

This imports two records. The first one is identified with the source IDs 1 and a.

The bit that can be customised is the ':' between the IDs, not the ',' between records.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes