update Atomic*::from_ptr and Atomic*::as_ptr docs

[usamoi]

Since #128778, it's allowed to perform atomic read and non-atomic read on the same atomic at the same time. Update the Atomic*::from_ptr and Atomic*::as_ptr documentation to remove expressions such as not allowed to mix atomic and non-atomic accesses.

see also std::sync::atomic

[rustbot]

r? @Mark-Simulacrum

rustbot has assigned @Mark-Simulacrum.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer