Confusion about upsert and delta functions in the docs

[aderchox]

With regards to the "Upsert" section on https://pouchdb.com/guides/conflicts.html and tell me what it's saying? I don't understand a word of it. E.g., What does it mean when it says "keep repeating put()"? Why would "repeating" put() resolve a conflict?
Also it says:

If the user's intended change can be expressed as a delta
How would the delta help for eventual consistency exactly? Is this like event sourcing?
And in the end, what's the role of this function? I don't understand it's useful for what at all:

function myDeltaFunction(doc) {
  doc.counter = doc.counter || 0;
  doc.counter++;
  return doc;
}

Thanks in advance.

[aderchox]

I've got it now! BUT the docs are really unclear / need more explanation in some parts, including this part, IMHO.
The upsert() retries doing put() "using a different _rev every time" (because it re-gets the document and then retries put()ing. (based on https://github.com/pouchdb/upsert/blob/9c781fc3f623c6f4622afa755b197ced1540c138/dist/pouchdb.upsert.js#L9)
Also the role of the delta function is providing the change as a function, rather than providing the changed document itself. (So that the change remains doable on newer revision of the document as well.)