"Field group" is overloaded and confusing in FieldTypeCategoryInterface

Problem/Motivation

While reviewing #3370326: Refine labels and descriptions for field types I raised some concerns in core/lib/Drupal/Core/Field/FieldTypeCategoryInterface.php around using the phrase "field group". E.g. the docs for getDescription() say "Returns the field group summary." In Drupal site building, when I see "field group", I think of https://www.drupal.org/project/field_group and/or an HTML fieldset.

Instead of holding up #3370326 any longer about this, opening a follow-up to explore new terminology and cleaning up the developer docs around this potentially confusing thing.

Steps to reproduce

Proposed resolution

TBD

Remaining tasks

User interface changes

Introduced terminology

TBD. Perhaps a notion of a "field category" would be better suited for this interface than "field group".

API changes

Data model changes

Release notes snippet