Add non-normative AMOCAS memory ordering clarification (riscv#1889)

In consultation with @ved-rivos.

Author: aswaterman

src/zacas.adoc

@@ -195,6 +195,8 @@ produced, the memory write access by an `AMOCAS.W/D/Q` instruction.
 An unsuccessful `AMOCAS.W/D/Q` may either not perform a memory write or may
 write back the old value loaded from memory. The memory write, if produced, does
 not have release semantics, regardless of `rl`.
+Irrespective of whether a write is actually performed, the instruction is
+treated as an AMO for the purposes of the RVWMO PPO rules.
 ====
 
 An `AMOCAS.W/D/Q` instruction always requires write permissions.