add a comment for the ctrl-u clear field combo

slock.c

@@ -185,7 +185,7 @@ readpw(Display *dpy, struct xrandr *rr, struct lock **locks, int nscreens,
 				    (len + num < sizeof(passwd))) {
 					memcpy(passwd + len, buf, num);
 					len += num;
-				} else if (buf[0] == '\025') {
+				} else if (buf[0] == '\025') { /* ctrl-u clears input */
 					explicit_bzero(&passwd, sizeof(passwd));
 					len = 0;
 				}