Adds explanations and auto filling for element body template

.../CoreBundle/Admin/ConfigurationFormContributionsAdmin.php

src/Biopen/CoreBundle/Admin/ConfigurationMapAdmin.php

src/Biopen/GeoDirectoryBundle/Admin/CategoryAdmin.php

src/Biopen/GeoDirectoryBundle/Admin/OptionAdmin.php

...ws/admin/core_custom/custom-fields/form-builder.html.twig

...urces/views/admin/core_custom/form_admin_fields.html.twig

...s/views/admin/partials/show_element_form_fields.html.twig