Docs: Add documentation and a `@since` note about the `loading` attribute...

Docs: Add documentation and a `@since` note about the `loading` attribute added to `wp_get_attachment_image()`.

Follow-up to [48272].

Props audrasjb.
Merges [48852] and [48853] to the 5.5 branch.
Fixes #51122.
Built from https://develop.svn.wordpress.org/branches/5.5@48854


git-svn-id: https://core.svn.wordpress.org/branches/5.5@48616 1a063a9b-81f0-0310-95a4-ce76da25c4cd