Improve documentation for aligned allocation on heap with registered arena and add contract tests

[OscarTHZhang]

For alignment allocations on heaps (mi_heap_malloc_aligned, mi_heap_zalloc_aligned, mi_heap_calloc_aligned, and mi_heap_realloc_aligned), there is an internal max limit on the alignment of 64 KiB. When the heap is from a registered arena, once the caller requests an alignment over this 64 KiB-limit, even if there is space available in the arena-backed heap, the request would fail and return a NULL pointer because it cannot fallback to OS-backed memory for over 64KiB alignment request.

This PR adds contract API tests to validate this behavior and improve the documentation over these functions to make it clear that there is a 64 KiB alignment limit when using with arena-backed heaps.

The issue is discussed in #1406