Improve purchase 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 purchasing type::feature + 1 deleted label

New review314641002 at https://codereview.tryton.org/314641002/#ps329311002

added 1 deleted label and removed 1 deleted label

review314641002 updated at https://codereview.tryton.org/314641002/#ps314671002

review314641002 updated at https://codereview.tryton.org/314641002/#ps329361004

review314641002 updated at https://codereview.tryton.org/314641002/#ps304921003

review314641002 updated at https://codereview.tryton.org/314641002/#ps322711002

review314641002 updated at https://codereview.tryton.org/314641002/#ps343191011

review314641002 updated at https://codereview.tryton.org/314641002/#ps357451002

New changeset b0a4025dc552 by David Harper in branch 'default':
Improve documentation
https://hg.tryton.org/modules/purchase/rev/b0a4025dc552

added 1 deleted label and removed 1 deleted label

closed

New changeset a3778b8c03cf by David Harper in branch 'default':
Improve documentation
https://hg.tryton.org/tryton-env/rev/a3778b8c03cf