memcpy and malloc_in_sandbox documentation

It would be useful to document this a bit better.

For memcpy: the last parameter the number of elements, right? The type makes it seem like that is indeed the case; since this is different from POSIX memcpy which takes the number of bytes, we should be clear about this.

For malloc: it's again the length of elements not bytes, right? (no argument = 1?)

I can update docs on ACK.