compiler_fence documentation: emphasize synchronization, not reordering
Our `fence` docs have at some point been update to explain that they are about synchronization, not about "preventing reordering". This updates the `compiler_fence` docs n the same vein, mostly by referring to the `fence` docs.
The old docs make it sound like I can put a compiler_fence in the middle of a bunch of non-atomic operations and that would achieve any kind of guarantee. It does not, atomic operations are still required to do synchronization.
I also slightly tweaked the `fence` docs, to put the synchronization first and the "prevent reordering" second.
Cc `@rust-lang/opsem` `@chorman0773` `@m-ou-se`
Fixes https://github.com/rust-lang/rust/issues/129189
Fixes https://github.com/rust-lang/rust/issues/54962
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