Add missing documentation for PurgeTask.

[avshenuk]

with segment deletion when no remaining records left:

• PEP Issue: [PEP] Enhance PurgeTask to automatically delete empty segments apache/pinot#16367
• PEP PR: feat: Enhance PurgeTask to automatically delete empty segments apache/pinot#16368

[avshenuk]

@xiangfu0 @Jackie-Jiang Wanted to add this new doc when I realized that there is no way to provide those custom implementations of RecordPurger and RecordModifier today from outside?

Then how could one even use this PurgeTask to begin with? What am I missing?

Edit: I could see that you could register them on MinionContext programmatically. But how would the end users do that?

[xiangfu0]

Can you try to set RecordPurgerFactory in the MinionContext?

[avshenuk]

Can you try to set RecordPurgerFactory in the MinionContext?

But at what stage can you grab that MinionContext and register yourself?

I mean, my class won't be loaded unless its name is explicitly provided like with stream decoders:

String decoderClass = _streamConfig.getDecoderClass();
    try {
      StreamMessageDecoder decoder = PluginManager.get().createInstance(decoderClass);
      decoder.init(fieldsToRead, _streamConfig, _tableConfig, _schema);
      return decoder;
    } catch (Exception e) {
      throw new RuntimeException(
          "Caught exception while creating StreamMessageDecoder from stream config: " + _streamConfig, e);
    }

But in this case there is nothing that can load my custom class? And so, there is no way for me to register myself in the MinionContext?
I'm so confused right now :)

Clearly people are using this task, but I'm really struggling to see how...

Edit: I guess one just need to provide a custom MinionStarter and register it there...