Update settings.rst with text

[alexmclarty]

Description of the Change

Add that REFRESH_TOKEN_EXPIRE_SECONDS can be an Int or datetime.timedelta to settings.rst.

Checklist

• PR only contains one change (considered splitting up PR)
• unit-test added
• documentation updated
• CHANGELOG.md updated (only for user relevant changes)
• author name in AUTHORS

[codecov]

Codecov Report

Merging #991 (e1365f1) into master (6625d3a) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #991   +/-   ##
=======================================
  Coverage   96.56%   96.56%           
=======================================
  Files          31       31           
  Lines        1716     1716           
=======================================
  Hits         1657     1657           
  Misses         59       59           

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