Improve account_product 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 accounting type::feature + 1 deleted label

New review322191002 at https://codereview.tryton.org/322191002/#ps316111002

review322191002 updated at https://codereview.tryton.org/322191002/#ps296391002

review322191002 updated at https://codereview.tryton.org/322191002/#ps329141002

added 1 deleted label and removed 1 deleted label

review322191002 updated at https://codereview.tryton.org/322191002/#ps295031002

review322191002 updated at https://codereview.tryton.org/322191002/#ps318841004

review322191002 updated at https://codereview.tryton.org/322191002/#ps331401002

review322191002 updated at https://codereview.tryton.org/322191002/#ps302841002

New changeset 3860aed8d26e by David Harper in branch 'default':
Improve documentation
https://hg.tryton.org/modules/account_product/rev/3860aed8d26e

added 1 deleted label and removed 1 deleted label

closed

New changeset e95d8772c13a by David Harper in branch 'default':
Improve documentation
https://hg.tryton.org/tryton-env/rev/e95d8772c13a