wp_register_custom_css_support: update PHP doc to clarify purpose

[ramonjd]

Follow up to:

• Block Supports: Strip custom CSS from blocks for users without edit_css capability #11347

Clarify the purpose of the wp_register_custom_css_support function.

See: https://core.trac.wordpress.org/ticket/64771#comment:65

"Removing styles.css" is technically correct, but the comment didn't explain the "why" in plain language.

Props to @audrasjb

Trac ticket: https://core.trac.wordpress.org/ticket/64771

[github-actions]

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[github-actions]

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Props ramonopoly, peterwilsoncc.

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[ramonjd]

Thanks @peterwilsoncc!

[ramonjd]

Committed in r62299 / 5d3de27