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

Thanks for creating this MR!
However, I see that the code already specifies everywhere (except for removeDatasource(), admittedly – funnily, that is the one method not mentioned in the issue title) that $datasource_id is a datasource plugin ID, in exactly those words. Therefore, I struggle to see what value these additional sentences provide. I’d very much like input from others before merging this.

One real problem I see in that area, though, which is not addressed by this issue is that setDatasources() apparently requires the $datasources array to be keyed by the datasource plugin IDs without specifiying this in the documentation.
Therefore we should either fix the documentation, or the code. (I think this would have blown up if anyone would have used this incorrectly so far, so I think changing the documentation would be fine here.)
Created 🐛 Fix contract of IndexInterface::setDatasources() Active as a follow-up for that. I’d just fix the minimal problem with the removeDatasource() docs in there unless enough people comment here to support expanding those doc comments.