Twenty Nineteen: Code Quality Improvements.

This change adds general code quality and documentation improvements.

More info here: https://github.com/WordPress/twentynineteen/pull/546 

Props grapplerulrich, iCaleb, allancole.

Merges [44187] to trunk.

See #45424.
Built from https://develop.svn.wordpress.org/trunk@44298


git-svn-id: https://core.svn.wordpress.org/trunk@44128 1a063a9b-81f0-0310-95a4-ce76da25c4cd