Updated the 'document download' instructions
Updated the 'document download' instructions

Tickets:
– Change title of "upload a document" in Python documentation: https://www.pivotaltracker.com/story/show/159524549
– Move info about getting document sending turned on for a service above all the implementation detail: https://www.pivotaltracker.com/story/show/159524401
– Add documentation about adding a personalisation variable into templates: https://www.pivotaltracker.com/story/show/159434598

Corrected a typo in the error codes for Letters
alphagov/notifications-python-client,alphagov/notifications-python-client