Fix NOTE about ARM FMA mnemonics (#1700)

h/t @ubc-guy

src/f-st-ext.adoc

@@ -311,8 +311,9 @@ product as the RISC-V instructions do, so the naming scheme was more
 rational at the time. The two definitions differ with respect to
 signed-zero results. The RISC-V definition matches the behavior of the
 x86 and ARM fused multiply-add instructions, but unfortunately the
-RISC-V FNMSUB and FNMADD instruction names are swapped compared to x86
-and ARM.
+RISC-V FNMSUB and FNMADD instruction names are swapped as compared to x86,
+whereas the RISC-V FMSUB and FNMSUB instruction names are swapped as
+compared to ARM.
 ====
 
 include::images/wavedrom/spfloat2.edn[]