Add note that presence of pack[w] implies presence of zext.h

src/b-st-ext.adoc

@@ -2386,6 +2386,13 @@ Included in::
 |Ratified
 |===
 
+NOTE: For RV32, the `pack` instruction with _rs2_=`x0` is the `zext.h`
+instruction.
+Hence, for RV32, any extension that contains the `pack` instruction also
+contains the `zext.h` instruction (but not necessarily the `c.zext.h`
+instruction, which is only guaranteed to exist if both the Zcb and Zbb
+extensions are implemented).
+
 <<<
 [#insns-packh,reftext="Pack low bytes of registers"]
 ==== packh
@@ -2484,6 +2491,13 @@ Included in::
 |Ratified
 |===
 
+NOTE: For RV64, the `packw` instruction with _rs2_=`x0` is the `zext.h`
+instruction.
+Hence, for RV64, any extension that contains the `packw` instruction also
+contains the `zext.h` instruction (but not necessarily the `c.zext.h`
+instruction, which is only guaranteed to exist if both the Zcb and Zbb
+extensions are implemented).
+
 <<<
 [#insns-rev8,reftext="Byte-reverse register"]
 ==== rev8