Document limitations of operator overloading utils

[adrhill]

Specify that the operator overloading guide is written for "Functions that take Real arguments".
This should clear up some confusion that popped up in #178 (comment).

[codecov-commenter]

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.36%. Comparing base (f0d36c6) to head (3cb5c77).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #180   +/-   ##
=======================================
  Coverage   91.36%   91.36%           
=======================================
  Files          41       41           
  Lines        1772     1772           
=======================================
  Hits         1619     1619           
  Misses        153      153           

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