xdg-user-dirs-update: add page (#3695)

pages/common/xdg-user-dirs-update.md

@@ -0,0 +1,12 @@
+# xdg-user-dirs-update
+
+> Update XDG user directories.
+> More information: <https://manpages.ubuntu.com/manpages/bionic/man1/xdg-user-dirs-update.1.html>.
+
+- Change XDG's DESKTOP directory to the specified directory (must be absolute):
+
+`xdg-user-dirs-update --set DESKTOP "{{/path/to/directory}}"`
+
+- Write the result to the specified dry-run-file instead of the `user-dirs.dirs` file:
+
+`xdg-user-dirs-update --dummy-output "{{/path/to/dry-run-file}}" --set {{xdg_user_directory}} "{{/path/to/directory}}"`