`ptr::copy_nonoverlapping`/`ptr::copy` docs mention order difference between memcpy/memmove but fail to note count unit difference

[thomcc]

The docs for ptr::copy_nonoverlapping and ptr::copy say things like:

copy is semantically equivalent to C's memmove, but with the argument order swapped

and

copy_nonoverlapping is semantically equivalent to C's memcpy, but with the argument order swapped.

This is true, but I just encountered code where these were more-or-less used as:

ptr::copy_nonoverlapping(a, b, size_of::<T>() * n);

and the author (and myself) were confused about the issue here due to this basically looking right. The problem here is that unlike memcpy/memmove the size_of::<T>() * n should just be n, since it's the number of Ts to copy.

I've filed a clippy lint for this: rust-lang/rust-clippy#6381, but the docs should be updated to clarify this difference.