Documentation: move a sentence about the type [production].

doc/main.tex

@@ -2187,6 +2187,9 @@ eventually be produced if the parser succeeds.
 \end{verbatim}
 
 The abstract type \verb+production+ represents a production of the grammar.
+%
+The ``start productions'' (which do not exist in an \mly file, but are
+constructed by Menhir internally) are \emph{not} part of this type.
 
 %% type 'a checkpoint
 
@@ -2487,10 +2490,6 @@ The function \verb+production_index+ converts a production to an integer
 number, whereas the function \verb+find_production+ carries out the reverse
 conversion. It is an error to apply \verb+find_production+ to an invalid
 index.
-%
-The ``start productions'' (which do not exist in an \mly file, but are
-constructed by Menhir internally) are \emph{not} part of the set of
-productions represented by the type \verb+production+.
 
 %% type element