Improve account 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 review292271005 at https://codereview.tryton.org/292271005/#ps296341003

added 1 deleted label and removed 1 deleted label

review292271005 updated at https://codereview.tryton.org/292271005/#ps329181002

review292271005 updated at https://codereview.tryton.org/292271005/#ps318511003

review292271005 updated at https://codereview.tryton.org/292271005/#ps326741002

review292271005 updated at https://codereview.tryton.org/292271005/#ps339311004

review292271005 updated at https://codereview.tryton.org/292271005/#ps302911002

New changeset 19a27fe2805a by David Harper in branch 'default':
Improve documentation
https://hg.tryton.org/modules/account/rev/19a27fe2805a

added 1 deleted label and removed 1 deleted label

closed

New changeset 278b6dc734ad by David Harper in branch 'default':
Improve documentation
https://hg.tryton.org/tryton-env/rev/278b6dc734ad