panel-title to card-title migration docs

Created by: Studio384

According to the migration docs for Bootstrap 4, .panel-title has been replaced with .card-title, however I think that's misleading because both classes have a different usage case. .panel-title was ment to be used within a .panel-heading (which was replaced with .card-header), whereas .card-title is - according to the docs - supposed to be used within a .card-block (the .panel-body replacement). Using .card-title within a .card-header gives unexpected results. It isn't a replacement, just a new element what a similar name but a totally different usage case. As it stands now, .panel-title is replaced with <h3 class="card-header">.