doc/main.tex: Precise that only unnamed semantic values can be referred using...

doc/main.tex: Precise that only unnamed semantic values can be referred using positional identifiers.

doc/main.tex

@@ -531,9 +531,10 @@ A semantic action is a piece of \ocaml code that is executed in order to
 assign a semantic value to the nonterminal symbol with which this production
 group is associated. A semantic action can refer to the (already computed)
 semantic values of the terminal or nonterminal symbols that appear in the
-production via the semantic value identifiers bound by the production. For
-compatibility with \ocamlyacc, semantic actions can also refer to these
-semantic values via positional keywords of the form
+production via the semantic value identifiers bound by the production.
+
+For compatibility with \ocamlyacc, semantic actions can also refer to
+unnamed semantic values via positional keywords of the form
 \kw{\$1}, \kw{\$2}, etc.\ This style is discouraged. Furthermore, as
 a positional keyword of the form \kw{\$i} is internally rewritten as
 \nt{\_i}, such identifiers are forbidden.