Clarify the behaviour of the `wp_editor_set_quality` and `jpeg_quality` filters.

Merges [30874] to the 4.1 branch.

Props DH-Shredder
See #29856

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Twenty Fifteen: remove license details for the image shown in the theme  screenshot file as it is Creative Commons Zero, which doesn't require attribution.

Merges [30867] to the 4.1 branch.

Props iamtakashi
See #30156

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