Twenty Seventeen: Improves code readability and code standards in files

Adds better DocBlock comments and fixes some spacing issues based on PHP_CodeSniffer WordPress coding standards.

Props sstoqnov.

Fixes #39152.

Built from https://develop.svn.wordpress.org/trunk@39618


git-svn-id: https://core.svn.wordpress.org/trunk@39558 1a063a9b-81f0-0310-95a4-ce76da25c4cd