Clarify MaybeUninit::zeroed padding docs

[JinRudy]

Closes #111608.

This clarifies that padding bytes in MaybeUninit::zeroed() are not guaranteed to remain zeroed when the MaybeUninit<T> value is returned, matching the existing mem::zeroed documentation that padding is not necessarily zeroed.

Verification:

• git diff --check
• ./x fmt --check
• ./x test tidy
• ./x test library/core --doc

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