wp_register_custom_css_support: update PHP doc to clarify purpose

Clarify the purpose of the `wp_register_custom_css_support` function. "Removing styles.css" is technically correct, but the comment didn't explain the "why" in plain language.

Props audrasjb, ramonopoly, peterwilsoncc.

Follow-up to [64771].



git-svn-id: https://develop.svn.wordpress.org/trunk@62299 602fd350-edb4-49c9-b593-d223f7449a82