Don't call init functions an extra time to validate them

[jgabry]

Submission Checklist

• Run unit tests
• Declare copyright holder and agree to license (see below)

Summary

When inits were specified using a function we were calling the function an extra time to validate it before using it, but we can just validate it at the time we use it.

Also made a few tiny edits to init argument doc.

Copyright and Licensing

Please list the copyright holder for the work you are submitting
(this will be you or your assignee, such as a university or company):
Jonah Gabry

By submitting this pull request, the copyright holder is agreeing to
license the submitted work under the following licenses:

• Code: BSD 3-clause (https://opensource.org/licenses/BSD-3-Clause)
• Documentation: CC-BY 4.0 (https://creativecommons.org/licenses/by/4.0/)

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.10%. Comparing base (595efe0) to head (e5c6c0c).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1195   +/-   ##
=======================================
  Coverage   91.10%   91.10%           
=======================================
  Files          15       15           
  Lines        6125     6125           
=======================================
  Hits         5580     5580           
  Misses        545      545           

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