Fix note about unaligned references in WasmRef docs

[chipshort]

Contrary to the current docs, WasmRef is not safe for unaligned references. This PR adjusts the docs.
Here's an example that shows the problem:

use wasmer::{Memory, MemoryType, Store, WasmPtr};

fn main() -> anyhow::Result<()> {
    let mut store = Store::default();
    let memory = Memory::new(&mut store, MemoryType::new(1, None, false))?;
    let memory = memory.view(&store);
    // write unaligned value into memory
    WasmPtr::<u32>::new(1).deref(&memory).write(42u32)?; // <- this panics with `misaligned pointer dereference`

    Ok(())
}

It would be possible to support this using write_unaligned, but that would require some refactoring, since it needs to be kept a pointer and not converted to a reference, as is currently done in WasmRefAccess::new.