Docs: Add missing documentation for the `$group` parameter of `WP_Object_Cache::get_multiple()`.

Synchronize documentation between `wp_cache_get_multiple()`, its compat version, and the class method.

See #20875.
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git-svn-id: https://core.svn.wordpress.org/trunk@47717 1a063a9b-81f0-0310-95a4-ce76da25c4cd