The value that can be passed to the `offset` parameter of `mi_malloc_aligned_at()`

Nobody has claimed this yet.

Code for passing a value that is not a multiple of the pointer size to the `offset` parameter of the `mi_malloc_aligned_at()` function and passing the returned pointer to the `mi_free()` function:
````c
#include "mimalloc.h"

int main(void) {
void *p = mi_malloc_aligned_at(50, 32, 4);
mi_free(p);
return 0;
}
````

in debug mode, you will get the following error message.
```
mimalloc: error: mi_free: invalid (unaligned) pointer: 0x2000000152f
````

Is it a specification that the value of the `offset` parameter must be a multiple of the pointer size?

Start with the mi_malloc_aligned_at() and mi_free() declarations and documentation in mimalloc.h, using the provided C example as the reproduction case. Determine whether a non-pointer-size-multiple offset is supported and document the expected offset and freeing behavior; the issue is done when the specification is explicit.

Written by the indexing model from the issue text.