Improve stock 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 stock type::feature + 1 deleted label

New review302311002 at https://codereview.tryton.org/302311002/#ps296531002

New review328331003 at https://codereview.tryton.org/328331003/#ps312541002

review302311002 updated at https://codereview.tryton.org/302311002/#ps328351002

review328331003 updated at https://codereview.tryton.org/328331003/#ps322271003

added 1 deleted label and removed 1 deleted label

review302311002 updated at https://codereview.tryton.org/302311002/#ps328801002

review302311002 updated at https://codereview.tryton.org/302311002/#ps316461003

review302311002 updated at https://codereview.tryton.org/302311002/#ps328941002

review302311002 updated at https://codereview.tryton.org/302311002/#ps331411002

review302311002 updated at https://codereview.tryton.org/302311002/#ps335381002

review302311002 updated at https://codereview.tryton.org/302311002/#ps333801002

review302311002 updated at https://codereview.tryton.org/302311002/#ps357101002

New changeset 0032a1985534 by David Harper in branch 'default':
Improve documentation
https://hg.tryton.org/modules/stock/rev/0032a1985534

added 1 deleted label and removed 1 deleted label

closed

New changeset 5c23b363f209 by David Harper in branch 'default':
Improve documentation
https://hg.tryton.org/tryton-env/rev/5c23b363f209

mentioned in commit 345c0ac9e173

mentioned in merge request !167 (merged)