Fix doc_cfg for FromZeroes for Option<Box<T>>

[joshlf]

zerocopy/src/lib.rs

Lines 1907 to 1908 in dcadae6

	#[cfg(feature = "alloc")]
	unsafe_impl!(T => FromZeroes for Option<Box<T>>);

This should have a #[cfg_attr(doc_cfg, doc(cfg(feature = "alloc")))] attribute, but putting it on the macro invocation won't result in it showing up in rustdoc (I tested it). You can see the result here. By contract, here's what it should look like.