doc: update Go memory model

Following discussion on #47141, make the following changes:

- Document Go's overall approach.
- Document that multiword races can cause crashes.
- Document happens-before for runtime.SetFinalizer.
- Document (or link to) happens-before for more sync types.
- Document happens-before for sync/atomic.
- Document disallowed compiler optimizations.

See also https://research.swtch.com/gomm for background.

Fixes #50859.

Change-Id: I17d837756a77f4d8569f263489c2c45de20a8778
Reviewed-on: https://go-review.googlesource.com/c/go/+/381315
Reviewed-by: Ian Lance Taylor <[email protected]>

Author: rsc