Document text orientations

The reference for the Transcription model does not include any documentation for text orientations, which definitely disoriented me when I worked on backend#1403 (closed), especially for vertical text. I could not find any past issue, wiki page, or documentation on what vertical-lr and vertical-rl meant. backend#850 (closed) only documented the meanings of the horizontal orientations.

You could use the API docs for the new centroid sorting as a starting point, so we don't end up having multiple definitions of the text orientations now…