Improve account_invoice 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

added 1 deleted label and removed 1 deleted label

New review308351002 at https://codereview.tryton.org/308351002/#ps304461003

review308351002 updated at https://codereview.tryton.org/308351002/#ps297031002

review308351002 updated at https://codereview.tryton.org/308351002/#ps313011002

review308351002 updated at https://codereview.tryton.org/308351002/#ps341161010

review308351002 updated at https://codereview.tryton.org/308351002/#ps330281002

New changeset cdef7fe6a80a by David Harper in branch 'default':
Improve documentation
https://hg.tryton.org/modules/account_invoice/rev/cdef7fe6a80a

added 1 deleted label and removed 1 deleted label

closed

New changeset 196b8a2e4ae2 by David Harper in branch 'default':
Improve documentation
https://hg.tryton.org/tryton-env/rev/196b8a2e4ae2