Update delete notes for Synrhconize docs (#3723)

The example for delete=yes does not specify recursive although it is
required. In addition, the wording for the delete option is confusing
about from where files are really deleted. This should clarify that.

files/synchronize.py

@@ -72,7 +72,7 @@ options:
     version_added: "1.5"
   delete:
     description:
-      - Delete files that don't exist (after transfer, not before) in the C(src) path. This option requires C(recursive=yes).
+      - Delete files in C(dest) that don't exist (after transfer, not before) in the C(src) path. This option requires C(recursive=yes).
     choices: [ 'yes', 'no' ]
     default: 'no'
     required: false
@@ -234,7 +234,7 @@ synchronize:
 delegate_to: "{{ inventory_hostname }}"
 
 # Synchronize and delete files in dest on the remote host that are not found in src of localhost.
-synchronize: src=some/relative/path dest=/some/absolute/path delete=yes
+synchronize: src=some/relative/path dest=/some/absolute/path delete=yes recursive=yes
 
 # Synchronize using an alternate rsync command
 # This specific command is granted su privileges on the destination