Clarify create_tables script

[jwkaltz]

Fixes #1563

See also comment in #1913 (comment)

[codecov]

Codecov Report

Attention: 4 lines in your changes are missing coverage. Please review.

Comparison is base (0e166c3) 80.59% compared to head (af422fc) 80.59%.

Files	Patch %	Lines
...reb/contrib/data_sources/standard/create_tables.py	0.00%	4 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1914   +/-   ##
=======================================
  Coverage   80.59%   80.59%           
=======================================
  Files         125      125           
  Lines        5266     5266           
=======================================
  Hits         4244     4244           
  Misses       1022     1022           

Flag	Coverage Δ
unittests	80.59% <0.00%> (ø)

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