gh-135171: Update documentation for the generator expression (GH-135351)

* gh-135171: Update documentation for the generator expression

Document that the iterator for the leftmost "for" clause is created
immediately.

* Update Doc/reference/expressions.rst

Co-authored-by: Brian Skinn <[email protected]>

---------

Co-authored-by: Brian Skinn <[email protected]>

Author: serhiy-storchaka

Doc/reference/expressions.rst

@@ -406,8 +406,9 @@ brackets or curly braces.
 Variables used in the generator expression are evaluated lazily when the
 :meth:`~generator.__next__` method is called for the generator object (in the same
 fashion as normal generators).  However, the iterable expression in the
-leftmost :keyword:`!for` clause is immediately evaluated, so that an error
-produced by it will be emitted at the point where the generator expression
+leftmost :keyword:`!for` clause is immediately evaluated, and the
+:term:`iterator` is immediately created for that iterable, so that an error
+produced while creating the iterator will be emitted at the point where the generator expression
 is defined, rather than at the point where the first value is retrieved.
 Subsequent :keyword:`!for` clauses and any filter condition in the leftmost
 :keyword:`!for` clause cannot be evaluated in the enclosing scope as they may

Misc/NEWS.d/next/Documentation/2025-06-10-17-02-06.gh-issue-135171.quHvts.rst

@@ -0,0 +1,2 @@
+Document that the :term:`iterator` for the leftmost :keyword:`!for` clause
+in the generator expression is created immediately.