gh-136640: clarify documentation of AST subclasses attributes

Author: Matthieu Lienart

Doc/library/ast.rst

@@ -91,11 +91,12 @@ Node classes
                   end_lineno
                   end_col_offset
 
-      Instances of :class:`ast.expr` and :class:`ast.stmt` subclasses have
-      :attr:`lineno`, :attr:`col_offset`, :attr:`end_lineno`, and
-      :attr:`end_col_offset` attributes.  The :attr:`lineno` and :attr:`end_lineno`
-      are the first and last line numbers of source text span (1-indexed so the
-      first line is line 1) and the :attr:`col_offset` and :attr:`end_col_offset`
+      Most instances of classes part of this module, e.g. instances of subclasses of
+      :class:`ast.expr`, :class:`ast.stmt` and others (see
+      `Abstract Grammar <abstract-grammar_>`__), have :attr:`lineno`, :attr:`col_offset`,
+      :attr:`end_lineno`, and :attr:`end_col_offset` attributes.  The :attr:`lineno` and
+      :attr:`end_lineno` are the first and last line numbers of source text span (1-indexed
+      so the first line is line 1) and the :attr:`col_offset` and :attr:`end_col_offset`
       are the corresponding UTF-8 byte offsets of the first and last tokens that
       generated the node. The UTF-8 offset is recorded because the parser uses
       UTF-8 internally.