Warning: Undefined index #description_display when adding a description to a fieldset group type

Problem/Motivation

When adding a description in a fieldset group type, a warning is launched: Warning: undefined index #description_display in form.inc line 217.

Repeatable: Always

Steps to repeat:
1. Download and install Module field_group
2. Add a new group or alter some previously created in the form display of an entity
3. Select fieldset as group type
4. Add a description for the group
5. Save the form display
6. Load the entity form
7. See the warnings in the top of the page

Expected Results:
Form loads without any warning

Actual Results:
A warning is showed: Warning: undefined index #description_display in form.inc line 217

Proposed resolution

The #description_display property is mandatory when a fieldset form element is created, but the field_group/src/Plugin/field_group/FieldGroupFormatter/Fieldset.php is not defining it.

I have created a patch that alters these formatter and adds description_display as a form setting property and defines the description_display value as after by default, so the warning is never launched. Also a new description_display select field was added to the group settings form so the user can select between after, before and invisible options to show the description.

Finally in the entity form settings config file, the description_display property is exported in the format settings section of the group.