Document possible values for enum types

[danischm]

In case a certain atttribute is of type enum, the possible values should be shown in the documentation. For example the "authentication_method" attribute of the "meraki_organizations_admins" resource only permits either "Cisco SecureX Sign-On" or "Email". Those allowed values should be shown in the documentation here: https://registry.terraform.io/providers/cisco-open/meraki/latest/docs/resources/organizations_admins#authentication_method