[IMP] doc: improve wording of guidelines introduction
[IMP] doc: improve wording of guidelines introduction

Purpose of this commit is to clean a bit the wording and to clarify how to
handle guidelines between stable and development versions.

ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo