Clarify behavior of upsert when document already exists

[xeger]

This almost caused an outage in production; thankfully I tested it, but I figured others might appreciate a how-to.

[codecov[bot]]

Codecov Report

Merging #621 (b623e2a) into master (ecf17e6) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #621   +/-   ##
=======================================
  Coverage   90.06%   90.06%           
=======================================
  Files          59       59           
  Lines        3020     3020           
=======================================
  Hits         2720     2720           
  Misses        300      300           

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[andrykonchin]

Thank you!