add a note about when .N is computed

[MichaelChirico]

Hoping to make clear in the documentation that .N in i may not be the same as .N in j. A subtle point inspired by #4695 -- if we allow all the symbols in both i and j, it might get a bit confusing to have them all "overloaded" in the sense that there's this subtle difference between what they mean in i vs j

[codecov]

Codecov Report

Merging #4696 (c710bb6) into master (a507ed5) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #4696   +/-   ##
=======================================
  Coverage   99.42%   99.42%           
=======================================
  Files          73       73           
  Lines       14455    14455           
=======================================
  Hits        14372    14372           
  Misses         83       83           

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