`clone_from_slice` doc can be updated after specialization

[bluss]

Current clone_from_slice doc says that the user might prefer to use copy_from_slice for Copy types, but since #81854 we automatically take care of this with specialization. We can consider to clarify docs.

The most careful action would be to let the current docs stand, if we don't want to say anything about the implementation details. I would prefer to remove the sentence about copy_from_slice, or something similar.

[the8472]

Do the docs currently admit any optimizations that happen through specialization? The only one that I can think of is the zero-optimization in vec!, and even that hedges with a "may be".