Update UnusedPrimitiveWarning text

[thehomebrewnerd]

Update UnusedPrimitiveWarning text

Closes #1141

Add clarifying text to UnusedPrimitiveWarning to explain behavior if multiple instances of a primitive are unused.

[codecov]

Codecov Report

Merging #2003 (8e3453c) into main (c682805) will increase coverage by 0.08%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #2003      +/-   ##
==========================================
+ Coverage   98.99%   99.07%   +0.08%     
==========================================
  Files         143      143              
  Lines       16369    16369              
==========================================
+ Hits        16204    16218      +14     
+ Misses        165      151      -14     

Impacted Files	Coverage Δ
featuretools/synthesis/dfs.py	100.00% <ø> (ø)
featuretools/tests/synthesis/test_dfs_method.py	100.00% <ø> (ø)
...computational_backends/calculate_feature_matrix.py	100.00% <0.00%> (+0.83%)	⬆️
...utational_backend/test_calculate_feature_matrix.py	100.00% <0.00%> (+1.19%)	⬆️

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[rwedge]

LGTM