add notes about alignment-altering reallocations to Allocator docs #95546
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As I said in rust-lang/wg-allocators#97, the fact that calls to
grow,grow_zeroed, andshrinkmay request altered alignments is surprising and may be a pitfall for implementors ofAllocatorif it's left implicit. This pull request adds a note to the "Safety" section of each function's docs making it explicit.