Change wording of 'directory' to 'folder' in docbooks, following what Frederik Schwarzer did

In the documentation, users may search for a string like "Compare Folders" (https://bugs.kde.org/show_bug.cgi?id=441380#c3 can be seen) but what it's written is "Compare Directories", so the documentation has to be changed, applying there what Frederik Schwarzer (@schwarzer) did on Change wording of Directory to Folder for user-visible strings.

That way, e.g. in the handbook: users will see texts (like "Compare Folders") as users see those texts in Krusader.