Missing docs and `Copy` impl for `Callback` #911

Job	Run time
get-leptos-changed / Detect Source Change	5s
CI (integrations/actix) / Run ci (nightly)	17m 46s
CI (integrations/axum) / Run ci (nightly)	17m 24s
CI (integrations/utils) / Run ci (nightly)	22m 34s
CI (integrations/viz) / Run ci (nightly)	18m 20s
CI (leptos) / Run ci (nightly)	1h 19m 25s
CI (leptos_dom) / Run ci (nightly)	28m 23s
CI (leptos_config) / Run ci (nightly)	16m 48s
CI (leptos_hot_reload) / Run ci (nightly)	15m 9s
CI (leptos_macro) / Run ci (nightly)	25m 54s
CI (leptos_reactive) / Run ci (nightly)	56m 39s
CI (leptos_server) / Run ci (nightly)	29m 47s
CI (meta) / Run ci (nightly)	16m 25s
CI (router) / Run ci (nightly)	17m 56s
CI (server_fn) / Run ci (nightly)	15m 56s
CI (server_fn_macro) / Run ci (nightly)	17m 42s
CI (server_fn/server_fn_macro_default) / Run ci (nightly)	19m 29s
	6h 55m 42s

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