Documentation and GUI fixes

Summary:
  I edited it to resolve some issues following the KDE Documentation style.
  - Important: change directory(ies) -> folder(s) (now is consistent)
    * Eight GUI messages where this change was still missing
  - GUI & Doc messages sync

Reviewers: yurchor, #documentation

Reviewed By: yurchor, #documentation

Subscribers: ognarb, kde-doc-english

Tags: #documentation

Differential Revision: https://phabricator.kde.org/D26084