docs(collections): add the word "unique" to distinctBy JSDoc for grepping purposes (#6335)

[lionel-rowe]

See #6335

Currently, grepping for "uniq" or "unique" yield zero results:

This PR is mainly to fix that problem, along with a note about how equality is determined.

Also:

• Changed name of callback param from selector to discriminator — "selector" is confusing as it gets the respective key from each item, rather than selecting items
• Changed example to something more representative of typical usage

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.43%. Comparing base (6cf1eea) to head (43d55fe).
Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6336   +/-   ##
=======================================
  Coverage   96.43%   96.43%           
=======================================
  Files         547      547           
  Lines       41788    41812   +24     
  Branches     6341     6347    +6     
=======================================
+ Hits        40297    40322   +25     
+ Misses       1450     1449    -1     
  Partials       41       41           

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[kt3k]

All suggestions look reasonable to me. Thanks for the PR!