clarify compiler_fence (and fence) docs

[RalfJung]

Also see rust-lang/libs-team#794 for more discussion.

Cc @Amanieu @m-ou-se

[rustbot]

r? @jhpratt

rustbot has assigned @jhpratt.
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

Why was this reviewer chosen?

The reviewer was selected based on:

• Owners of files modified in this PR: libs
• libs expanded to 8 candidates
• Random selection from Mark-Simulacrum, jhpratt

[RalfJung]

To give an example where saying "it prevents reordering" is failing to capture some nuance:

// ptr: *mut i32

ptr.write(0);
fence(Release);
ptr.write(1);
fence(Release);

In this example, the compiler is totally allowed to reorder the first write down across the release fence. (In fact the compiler is allowed to entirely remove the first fence, it is subsumed by the second fence.)

View changes since the review