Documentation: clarified that a nonterminal symbol should begin with a

lowercase letter.

doc/main.tex

@@ -498,8 +498,14 @@ the parser.
 \subsection{Rules}
 
 Following the mandatory \percentpercent keyword, a sequence of rules is
-expected. Each rule defines a nonterminal symbol~\nt{id}. In its simplest
-form, a rule begins with \nt{id}, followed by a colon character (\deuxpoints),
+expected. Each rule defines a nonterminal symbol~\nt{id}.
+%
+(It is recommended that the name of a nonterminal symbol begin with a lowercase
+letter, so it falls in the category \nt{lid}. This is in fact mandatory for the
+start symbols.)
+In its simplest
+form, a rule begins with the nonterminal symbol \nt{id},
+followed by a colon character (\deuxpoints),
 and continues with a sequence of production groups
 (\sref{sec:productiongroups}). Each production group is preceded with a
 vertical bar character (\barre); the very first bar is optional. The meaning