chore(backport release-1.1): docs: document exprlang func return values

[akuitybot]

Automated backport to release-1.1, triggered by a label in #3263.

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 51.17%. Comparing base (2189b9a) to head (b3c7406).
Report is 1 commits behind head on release-1.1.

Additional details and impacted files

@@             Coverage Diff              @@
##           release-1.1    #3264   +/-   ##
============================================
  Coverage        51.17%   51.17%           
============================================
  Files              285      285           
  Lines            25627    25627           
============================================
  Hits             13115    13115           
  Misses           11814    11814           
  Partials           698      698           

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