minor touch-up and documentation

as I was playing with it to understand what those logics mean.

core/src/main/java/hudson/scheduler/CronTab.java

@@ -443,13 +443,15 @@ public final class CronTab {
      * @since XXX
      */
     public static @CheckForNull String hashify(String spec) {
-        if (spec.startsWith("*/")) {
+        if (spec.contains("H")) {
+            // if someone is already using H, presumably he knows what it is, so a warning is likely false positive
+            return null;
+        } else if (spec.startsWith("*/")) {// "*/15 ...." (every N minutes) to hash
             return "H" + spec.substring(1);
-        } else if (spec.matches("\\d+ .+")) {
+        } else if (spec.matches("\\d+ .+")) {// "0 ..." (certain minute) to hash
             return "H " + spec.substring(spec.indexOf(' ') + 1);
         } else {
             return null;
         }
     }
-
 }

core/src/test/java/hudson/scheduler/CronTabTest.java

@@ -187,6 +187,8 @@ public class CronTabTest {
         assertEquals(Messages.CronTab_spread_load_evenly_by_using_rather_than_("H * * * *", "0 * * * *"), new CronTab("0 * * * *").checkSanity());
         // if the user specifically asked for 3:00 AM, probably we should stick to 3:00–3:59
         assertEquals(Messages.CronTab_spread_load_evenly_by_using_rather_than_("H 3 * * *", "0 3 * * *"), new CronTab("0 3 * * *").checkSanity());
+
+        assertEquals(null, new CronTab("H/15 * 1 1 *").checkSanity());
     }
 
     /**