treewide: fix typos

- mostly: usecase -> use case
- continously -> continuously
- single typos in docs/FILE_DESCRIPTOR_STORE.md

src/tmpfiles/tmpfiles.c

@@ -3278,7 +3278,7 @@ static int patch_var_run(const char *fname, unsigned line, char **path) {
          * might create the paths that are intermediary to the listed paths. We can't really cover the generic case,
          * but the least we can do is cover the specific case of /var/run vs. /run, as /var/run is a legacy name for
          * /run only, and we explicitly document that and require that on systemd systems the former is a symlink to
-         * the latter. Moreover files below this path are by far the primary usecase for tmpfiles.d/. */
+         * the latter. Moreover files below this path are by far the primary use case for tmpfiles.d/. */
 
         k = path_startswith(*path, "/var/run/");
         if (isempty(k)) /* Don't complain about other paths than /var/run, and not about /var/run itself either. */