update-done: include a short description in .updated

People might be confused where the file comes from, since the name is not
at all specific.

https://bugzilla.redhat.com/show_bug.cgi?id=1121301#c8

src/update-done/update-done.c

@@ -22,6 +22,11 @@
 #include "util.h"
 #include "label.h"
 
+#define MESSAGE                                                         \
+        "This file was created by systemd-update-done. Its only \n"     \
+        "purpose is to hold a timestamp of the time this directory\n"   \
+        "was updated. See systemd-update-done.service(8).\n"
+
 static int apply_timestamp(const char *path, struct timespec *ts) {
         struct timespec twice[2];
         struct stat st;
@@ -77,6 +82,8 @@ static int apply_timestamp(const char *path, struct timespec *ts) {
                         return -errno;
                 }
 
+                (void) loop_write(fd, MESSAGE, strlen(MESSAGE), false);
+
                 twice[0] = *ts;
                 twice[1] = *ts;