Clarify note about calling convention

For greater clarity, this commit moves the comment about the calling
convention for directionless parameters of extern object type out of
an itemized list. Instead, it is a standalone sentence below the list.

p4-16/spec/P4-16-spec.mdk

@@ -1506,8 +1506,7 @@ Each parameter may be labeled with a direction:
   - For anything other than an action, e.g. a control, parser, or
     function, a directionless parameter means that the value supplied
     as an argument in a call must be a compile-time known value
-    (see Section [#sec-compile-time-known]). Note that a directionless 
-    parameter of extern object type is passed by reference.
+    (see Section [#sec-compile-time-known]).
   - For an action, a directionless parameter indicates that it is
     "action data".  See Section [#sec-actions] for the meaning of
     action data, but its meaning includes the following possibilities:
@@ -1518,6 +1517,8 @@ Each parameter may be labeled with a direction:
       when an entry is added to a table that uses that action.  See
       Section [#sec-actions].
 
+A directionless parameter of extern object type is passed by reference.
+
 Direction `out` parameters are always initialized at the beginning of
 execution of the portion of the program that has the `out` parameters,
 e.g. `control`, `parser`, `action`, function, etc.  This