Rollup merge of #120823 - LegionMammal978:clarify-atomic-align, r=RalfJung

Clarify that atomic and regular integers can differ in alignment

The documentation for atomic integers says that they have the "same in-memory representation" as their underlying integers. This might be misconstrued as implying that they have the same layout. Therefore, clarify that atomic integers' alignment is equal to their size.