Improve 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 product type::feature + 1 deleted label

New review308231002 at https://codereview.tryton.org/308231002/#ps300551002

added 1 deleted label and removed 1 deleted label

review308231002 updated at https://codereview.tryton.org/308231002/#ps312471002

review308231002 updated at https://codereview.tryton.org/308231002/#ps328751007

review308231002 updated at https://codereview.tryton.org/308231002/#ps318861002

review308231002 updated at https://codereview.tryton.org/308231002/#ps335201002

review308231002 updated at https://codereview.tryton.org/308231002/#ps302841003

review308231002 updated at https://codereview.tryton.org/308231002/#ps339481002

New changeset 4fdb6a6de540 by David Harper in branch 'default':
Improve documentation
https://hg.tryton.org/modules/product/rev/4fdb6a6de540

added 1 deleted label and removed 1 deleted label

closed

New changeset 26b1e6ec6024 by David Harper in branch 'default':
Improve documentation
https://hg.tryton.org/tryton-env/rev/26b1e6ec6024

New changeset f1ad83824aa6 by David Harper in branch 'default':
Add improved documentation files
https://hg.tryton.org/modules/product/rev/f1ad83824aa6

New changeset a9d7310e38ac by David Harper in branch 'default':
Add improved documentation files
https://hg.tryton.org/tryton-env/rev/a9d7310e38ac