gh-135110: Fix misleading `generator.close()` documentation (GH-135152)

gh-135110: Fix misleading `generator.close()` documentation (GH-135152)

The documentation incorrectly stated that generator.close() 'raises' a
GeneratorExit exception. This was misleading because the method doesn't
raise the exception to the caller - it sends the exception internally
to the generator and returns None.