clarify ACPERM rules (#421)

fix #415

src/insns/acperm_32bit.adoc

@@ -62,7 +62,8 @@ The MXLEN=32 additional rules are:
 . Clear <<asr_perm>> unless _all_ other permissions are set
 . Clear <<c_perm>> and <<x_perm>> if <<r_perm>> is not set
 . Clear <<w_perm>> if <<c_perm>> _is_ set, but <<lm_perm>> _is not_ set.
-. Clear <<x_perm>> if <<x_perm>> and <<r_perm>> _are_ set, but <<c_perm>> and <<w_perm>> _are not_ set.
+. Clear <<x_perm>> if <<x_perm>> and <<r_perm>> are set, and <<c_perm>> and <<w_perm>> are both clear.
+. If removing <<lm_perm>> results in unrepresentable permissions, then set <<r_perm>> and <<c_perm>> only.
 
 .Capability permissions bit field
 [#acperm_bit_field]