Customize: Clean up docs and code style for customize changes in 4.7.

* Adds missing `resolve`/`fail` for promise returned by `loadThemePreview`.
* Adds missing jsdoc blocks and tags.
* Adds missing phpdoc and makes corrections.

See #37770.
Fixes #38908.

Built from https://develop.svn.wordpress.org/trunk@39345


git-svn-id: https://core.svn.wordpress.org/trunk@39285 1a063a9b-81f0-0310-95a4-ce76da25c4cd