Improve country 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 country ~844 + 1 deleted label

New review312311002 at https://codereview.tryton.org/312311002/#ps324021002

review312311002 updated at https://codereview.tryton.org/312311002/#ps328071006

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review312311002 updated at https://codereview.tryton.org/312311002/#ps329071003

review312311002 updated at https://codereview.tryton.org/312311002/#ps322471002

review312311002 updated at https://codereview.tryton.org/312311002/#ps320361002

review312311002 updated at https://codereview.tryton.org/312311002/#ps314761002

review312311002 updated at https://codereview.tryton.org/312311002/#ps331111002

review312311002 updated at https://codereview.tryton.org/312311002/#ps331301003

review312311002 updated at https://codereview.tryton.org/312311002/#ps335181002

review312311002 updated at https://codereview.tryton.org/312311002/#ps343211002

review312311002 updated at https://codereview.tryton.org/312311002/#ps339371002

New changeset 0a79e3d9bc2a by David Harper in branch 'default':
Improve documentation
https://hg.tryton.org/modules/country/rev/0a79e3d9bc2a

added 1 deleted label and removed 1 deleted label

closed

New changeset acee3d7bb1c0 by David Harper in branch 'default':
Improve documentation
https://hg.tryton.org/tryton-env/rev/acee3d7bb1c0