[3.14] gh-135171: Update documentation for the generator expression (GH-135351) (#135524)

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



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(cherry picked from commit 8979d3afe376c67931665070a79f6939ebcd940b)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>
Co-authored-by: Brian Skinn <brian.skinn@gmail.com>