Note limited meaning of box in assembly

[padix-key]

Notes that the Box attribute of an AtomArray has limited meaning in get_assembly() as discussed in #888.

[codspeed-hq]

Merging this PR will not alter performance

✅ 98 untouched benchmarks

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_{Comparing padix-key:issue-888 (c5c57af) with main (c236d1a)}