Clarify MCMC-nuts chain argument documentation for mcmc_nuts_stepsize().

[BjarkeHautop]

The default behavior for mcmc_nuts_stepsize() and what the argument chain does is different than for the rest. Updated the documentation to reflect this.

[jgabry]

Thanks for fixing this. I hadn't noticed this.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.61%. Comparing base (5a5b1e5) to head (5b46c93).
⚠️ Report is 3 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #413   +/-   ##
=======================================
  Coverage   98.61%   98.61%           
=======================================
  Files          35       35           
  Lines        5792     5792           
=======================================
  Hits         5712     5712           
  Misses         80       80           

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