Improve sale module documentation

Documentation improvements as part of the GSoD: https://discuss.tryton.org/t/gsod-project-documentation-for-basic-modules/3186

assigned to @dave

added sales type::feature + 1 deleted label

New review328601002 at https://codereview.tryton.org/328601002/#ps328611002

added 1 deleted label and removed 1 deleted label

review328601002 updated at https://codereview.tryton.org/328601002/#ps302641002

review328601002 updated at https://codereview.tryton.org/328601002/#ps304941002

review328601002 updated at https://codereview.tryton.org/328601002/#ps339101002

review328601002 updated at https://codereview.tryton.org/328601002/#ps292941002

review328601002 updated at https://codereview.tryton.org/328601002/#ps340551002

review328601002 updated at https://codereview.tryton.org/328601002/#ps330721002

New changeset 3eef9f032cfb by David Harper in branch 'default':
Improve documentation
https://hg.tryton.org/modules/sale/rev/3eef9f032cfb

added 1 deleted label and removed 1 deleted label

closed

New changeset ce7661a88f02 by David Harper in branch 'default':
Improve documentation
https://hg.tryton.org/tryton-env/rev/ce7661a88f02

marked #9338 (closed) as a duplicate of this issue

marked this issue as related to #9338 (closed)