[IMP] core: docstring improvements

* clean and improve docstrings in orm
* fix typos found with codespell
* rely on the Environment class docstring instead of doc content (and 
therefore move part of the doc inside the class docstring)

closes odoo/odoo#102896

Related: odoo/documentation#2798
Signed-off-by: Victor Feyens (vfe) <vfe@odoo.com>