Add clarification that misa implications are one-way

src/machine.adoc

@@ -228,6 +228,11 @@ write to `misa` is suppressed, leaving `misa` unchanged.
 When software enables an extension that was previously disabled, then
 all state uniquely associated with that extension is UNSPECIFIED, unless otherwise specified by that extension.
 
+NOTE: Although one of the bits 25--0 in `misa` being set to 1 implies that
+the corresponding feature is implemented, the inverse is not necessarily
+true: one of these bits being clear does not necessarily imply that the
+corresponding feature is absent.
+
 ==== Machine Vendor ID Register `mvendorid`
 
 The `mvendorid` CSR is a 32-bit read-only register providing the JEDEC