Clarify is_unconstrained docstring

[penelopeysm]

I find the double negatives confusing.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.16%. Comparing base (727da63) to head (244ccf2).
Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #790   +/-   ##
=======================================
  Coverage   86.16%   86.16%           
=======================================
  Files          36       36           
  Lines        4301     4301           
=======================================
  Hits         3706     3706           
  Misses        595      595           

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[coveralls]

Pull Request Test Coverage Report for Build 12975008585

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 86.246%

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Totals
Change from base Build 12908348905:	0.0%
Covered Lines:	3706
Relevant Lines:	4297

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💛 - Coveralls

[torfjelde]

LGTM!