IndexInterface methods getDatasourceIds(), isValidDatasource(), getDatasource() should document what a datasource ID is

Problem/Motivation

AFAICT the $datasource_id parameter to these methods in IndexInterface is just a datasource plugin ID. This makes sense, as a datasource can only be added once to an index.

These methods should document that the $datasource_id is the datasource plugin ID.

Steps to reproduce

Proposed resolution

Remaining tasks