gh-156866: Document that pack_into() does not guarantee memory access shape

[lazerg]

struct.pack_into() only documents which bytes end up in the buffer, not how the write happens underneath. On ordinary memory that distinction never matters, but on a buffer backed by memory-mapped I/O the number, width, and ordering of the actual stores can be significant, and a single pack_into() call isn't guaranteed to turn into one native store. This adds a short note to that effect so nobody assumes pack_into() is a register-width MMIO write primitive.

Fixes #156866

• Issue: Documentation: clarify that struct.pack_into() does not guarantee exact memory-access semantics #156866

[read-the-docs-community]

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