Explicitly mention in the docs that `grow`, `grow_zeroed`, and `shrink` have to handle changes in alignment

The docs currently imply that the alignment of `new_layout` may be different from that of `old_layout` by not stating otherwise. Reallocations with altered alignments are extremely uncommon; I had to track down #5 in this repository to convince myself this wasn't a mistake. This could be a pitfall for implementors of `Allocator` if it's not made more explicit.