rework predict docstring

[palday]

Thanks for contributing!

Did behavior change? Did you add need features? If so, please update NEWS.md

• add entry in NEWS.md
• after opening this PR, add a reference and run docs/NEWS-update.jl to update the cross-references.

No behavioral changes, but we updated a docstring enough that a patch release might be worthwhile.

Should we release your changes right away? If so, bump the version:

• I've bumped the version appropriately

@kliegl is the new explanation clearer?

[codecov]

Codecov Report

Merging #603 (96b82aa) into main (3d868bb) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #603   +/-   ##
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  Coverage   96.36%   96.36%           
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  Files          28       28           
  Lines        2695     2695           
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  Hits         2597     2597           
  Misses         98       98           

Impacted Files	Coverage Δ
src/predict.jl	98.33% <ø> (ø)

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[ararslan]

Two minor things, otherwise LGTM.

[palday]

@ararslan as someone who uses MixedModels but didn't write the code in question, do the descriptions of the options for new_re_levels make sense ?

[kliegl]

@palday How about including a sentence: Random effects (not only fixed effects) are included for predictions involving grouping variables with observed levels.