docs: Update doctext sample to include method signature (#275)
docs: Update doctext sample to include method signature (#275)

* docs: update async_detect_document() sample

* docs: Update doctext sample to include method signature

Co-authored-by: Anthonios Partheniou <50de7473c4bcd6fb06c46bebfdc91e30df56824a@google.com>
googleapis/python-vision,googleapis/python-vision