qapi: Belatedly adjust limitations documentation

Commit 57df0dff1a "qapi: Extend -compat to set policy for unstable
interfaces" (v6.2.0) took care of covering experimental features, but
neglected to adjust a comment suggesting to cover it.  Adjust it now.

Fixes: 57df0dff1a
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20220225084538.218876-1-armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>

qapi/compat.json

@@ -41,7 +41,7 @@
 #
 # Limitation: covers only syntactic aspects of QMP, i.e. stuff tagged
 # with feature 'deprecated'.  We may want to extend it to cover
-# semantic aspects, CLI, and experimental features.
+# semantic aspects and CLI.
 #
 # Limitation: deprecated-output policy @hide is not implemented for
 # enumeration values.  They behave the same as with policy @accept.