Docs: Improve documentation for the `$attr` parameter of `wp_get_attachment_image()`.

Props juanfra, Mte90, DrewAPicture, joemcgill, SergeyBiryukov.
Fixes #36661.
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git-svn-id: https://core.svn.wordpress.org/trunk@47049 1a063a9b-81f0-0310-95a4-ce76da25c4cd