Add some explanation to sync_batch_stats

[marcvanzee]

The x argument was unclear to a user so it may be useful to explain it in a comment.

[google-cla]

All (the pull request submitter and all commit authors) CLAs are signed, but one or more commits were authored or co-authored by someone other than the pull request submitter.

We need to confirm that all authors are ok with their commits being contributed to this project. Please have them confirm that by leaving a comment that contains only @googlebot I consent. in this pull request.

Note to project maintainer: There may be cases where the author cannot leave a comment, or the comment is not properly detected as consent. In those cases, you can manually confirm consent of the commit author(s), and set the cla label to yes (if enabled on your project).

ℹ️ Googlers: Go here for more info.

[codecov-io]

Codecov Report

Merging #871 (f4d234b) into master (7cdfcaf) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #871   +/-   ##
=======================================
  Coverage   80.83%   80.83%           
=======================================
  Files          55       55           
  Lines        4389     4389           
=======================================
  Hits         3548     3548           
  Misses        841      841           

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 7cdfcaf...f4d234b. Read the comment docs.