Improve party module documentation

Improve documentation along the lines described in
https://discuss.tryton.org/t/documentation-on-the-proper-place/2414

assigned to @dave

added party type::feature + 1 deleted label

added 1 deleted label and removed 1 deleted label

New review297241003 at https://codereview.tryton.org/297241003/#ps299141002

review297241003 updated at https://codereview.tryton.org/297241003/#ps297281002

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