[ABNF] Improve formulation of rule for types.

As discussed on GitHub, list identifiers separately from Self.

Also adapt and improve some of the documentation comments.

grammar/abnf-grammar.txt

@@ -673,21 +673,28 @@ array-type = "[" type ";" array-dimensions "]"
 array-dimensions = natural
                  / "(" natural *( "," natural ) ")"
 
-; Circuit types are denoted by identifiers and the keyword `Self`;
-; the latter is only allowed inside a circuit definition,
-; to denote the circuit type being defined.
+; The keyword `Self` denotes the enclosing circuit type.
+; It is only allowed inside a circuit type declaration.
+
+self-type = %s"Self"
+
+; Circuit types are denoted by identifiers and by `Self`.
 ; Identifiers may also be type aliases;
 ; syntactically (i.e. without a semantic analysis),
 ; they cannot be distinguished from circuit types.
 
-self-type = %s"Self"
+; Scalar types, tuple types, array types,
+; identifiers (which may be circuit types or type aliases),
+; and the `Self` type
+; form all the types.
+
+type = scalar-type / tuple-type / array-type / identifier / self-type
+
+; It is convenient to introduce a rule for types that are
+; either identifiers or `Self`, as this is used in other rules.
 
 identifier-or-self-type = identifier / self-type
 
-; Scalar and the remaining types form all the types.
-
-type = scalar-type / tuple-type / array-type / identifier-or-self-type
-
 ; The lexical grammar given earlier defines product group literals.
 ; The other kind of group literal is a pair of integer coordinates,
 ; which are reduced modulo the prime to identify a point,