h/t Marc

Resolves #449

h/t @jhauser-us

As the top of the section states, not all AMOs need be supported by
all of main memory.

h/t @jhauser-us

On behalf of @jhauser-us

Representing only "virtual addresses" suffices, because the physical
addresses that the PC can hold or that loads and stores can use when
address translation is disabled _are_ virtual addresses.

There's no reason for simple pedagogical implementations to trap
FENCE; they can probably just execute it as a NOP.  (In situations
where that statement is false, the implementation isn't simple!)

I think the sentence was written with FENCE.I in mind, but since that
instruction has been moved to an extension, the sentence doesn't apply
in this context anymore.

h/t @noahsherrill with an assist from @daniellustig

Closes #443

Resolves #426

These are now available at https://github.com/riscv/riscv-isa-manual/releases/tag/archive