[SPARK-5984]Add notes and fix puzzling notes

[xiaoyesoso]

What changes were proposed in this pull request?

Notes

How was this patch tested?

(Please explain how this patch was tested. E.g. unit tests, integration tests, manual tests)
(If this patch involves UI changes, please attach a screenshot; otherwise, remove this)

Please review http://spark.apache.org/contributing.html before opening a pull request.

[AmplabJenkins]

Can one of the admins verify this patch?

[srowen]

This has no explanation. This is also copied from the Android OSS project, so I don't think I'd change the comments without reason. It'd be easier to update later if needed

[xiaoyesoso]

@srowen
The explanations are for TimSort from Android OSS。The code has bug (see it )

private void mergeCollapse() {
  while (stackSize > 1) {
    int n = stackSize - 2;
    if (n > 0 && runLen[n-1] <= runLen[n] + runLen[n+1]) {
      if (runLen[n - 1] < runLen[n + 1])
         n--;
      mergeAt(n);
    } else if (runLen[n] <= runLen[n + 1]) {
      mergeAt(n);
    } else {
      break; // Invariant is established
    }
  }
}

But we have fixed it. (see SPARK-5984) .So leaving these explanations may puzzling.

[srowen]

OK, but you're un-documenting an invariant that is still supposed to be true. Your new invariant is the same as the first too.