Docs: Add `@since` tag for the `$context` parameter added to...

Docs: Add `@since` tag for the `$context` parameter added to `wp_generate_attachment_metadata` filter in [46621].

See #48472.
Built from https://develop.svn.wordpress.org/trunk@46622


git-svn-id: https://core.svn.wordpress.org/trunk@46419 1a063a9b-81f0-0310-95a4-ce76da25c4cd