[doc]edpm_pre_adoption_validation

Adds description for the role

Author: gibizer

docs/source/roles/role-edpm_pre_adoption_validation.rst

@@ -2,5 +2,15 @@
 Role - edpm_pre_adoption_validation
 ===================================
 
+Implements a set of checks to be run before the main edpm_ansible deployment
+executed during adoption but after the new OpenStackDataPlaneNodeSet CRs are
+fully configured. See
+[the adoption docs](https://openstack-k8s-operators.github.io/data-plane-adoption/)
+for the detailed steps.
+
+The goals of these checks to prevent applying such
+new configuration to the edpm nodes that would either cause immediate problems
+of the running workload or require a reboot to apply.
+
 .. include::
   ../collections/osp/edpm/edpm_pre_adoption_validation_role.rst