docs: add comprehensive documentation for borrowing from repr(packed)…

[hns1971]

Document borrowing rules for repr(packed) fields (E0793).

This documents the behavior implemented in rust-lang/rust#145419:

• Disallow borrowing when the target’s ABI alignment exceeds the packed alignment (hard error E0793).
• Allow borrowing when ABI alignment is ≤ the packed alignment.
• For [T; N]/[T], alignment equals that of T, independent of N.

Includes examples for the allowed [u8; N] case and the disallowed [u16; N] case.