Add tab_caption()

[billdenney]

Summary

Add tab_caption(). Also, tab_caption() is used where other, less-specific calls had been used before.

Related GitHub Issues and PRs

• Ref: Fix #1048

Checklist

• [x] I understand and agree to the Code of Conduct.
• [x] I have listed any major changes in the NEWS.
• [x] I have added testthat unit tests to tests/testthat for any new functionality.