atomic.fence is mentioned in the Overview but not in the spec

[jakobkummerow]

The atomic.fence instruction was added to the Overview, but neither to the binary nor text format spec.

(Looks like this plan fell through the cracks.)

Related note: I find it surprising that the text format uses memory.atomic.wait32 and memory.atomic.wait64 instead of i32.atomic.wait and i64.atomic.wait (like the other instructions), but I totally don't feel strongly about it, so if this is something that's been discussed and decided before then please just ignore me :-)

[lars-t-hansen]

Re naming, there was a bikeshedding session:
#145
#149

Related to fence:
#143