Minimize uvars test

[newville]

This updates the test and docs to better explain how to use MinimizerResult.uvars.

Type of Changes

• Bug fix
• New feature
• Refactoring / maintenance
• Documentation / examples

Tested on

Verification

Have you

• included docstrings that follow PEP 257?

• referenced existing Issue and/or provided relevant link to mailing list?

• verified that existing tests pass locally?

• verified that the documentation builds locally?

• squashed/minimized your commits and written descriptive commit messages?

• added or updated existing tests to cover the changes?
• updated the documentation and/or added an entry to the release notes (doc/whatsnew.rst)?
• added an example?

[codecov]

Codecov Report

Merging #913 (a3c55c2) into master (261ce28) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #913   +/-   ##
=======================================
  Coverage   93.23%   93.23%           
=======================================
  Files          10       10           
  Lines        3696     3696           
=======================================
  Hits         3446     3446           
  Misses        250      250