Specify that value_set's size is compile-time known

Signed-off-by: Jaehyun Lee <[email protected]>

p4-16/spec/P4-16-spec.adoc

@@ -6565,8 +6565,8 @@ include::grammar.adoc[tag=valueSetDeclaration]
 ----
 
 Parser Value Sets support a `size` argument to provide hints to the compiler to
-reserve hardware resources to implement the value set. For example, this parser
-value set:
+reserve hardware resources to implement the value set. Thus, the `size` argument
+must be a compile-time known value. For example, this parser value set:
 
 [source,p4]
 ----