Docment that --max-delete=0 now works.

NEWS

 NEWS for rsync 3.0.0 (UNRELEASED)
-Protocol: 29 (unchanged)
+Protocol: 30 (changed)
 Changes since 2.6.9:
 
   BUG FIXES:
@@ -13,7 +13,11 @@ Changes since 2.6.9:
 
   ENHANCEMENTS:
 
-    - ...
+    - You may specify --max-delete=0 to a 3.0.0 client as long as the
+      receiving side is at least version 3.0.0.  This means that you
+      can pull from an older rsync with this option, but pushing to an
+      older rsync will generate an error.  *Be careful to never specify
+      a 0 value to older rsync client, or it will be silently ignored.*
 
   INTERNAL:
 

rsync.yo

@@ -1004,7 +1004,12 @@ using bf(--delete-after), and it used to be non-functional unless the
 bf(--recursive) option was also enabled.
 
 dit(bf(--max-delete=NUM)) This tells rsync not to delete more than NUM
-files or directories (NUM must be non-zero).
+files or directories.
+Beginning with version 3.0.0, you may specify bf(--max-delete=0) to
+be warned about any extraneous files in the destination, but be very
+careful to never specify a 0 value to an older rsync client, or the
+option will be silently ignored.  (A 3.0.0 client will die with an
+error if the remote rsync is not new enough to handle the situation.)
 This is useful when mirroring very large trees to prevent disasters.
 
 dit(bf(--max-size=SIZE)) This tells rsync to avoid transferring any