updating doc to include revoke example for mysql_user docs

library/database/mysql_user

@@ -123,6 +123,9 @@ EXAMPLES = """
 # Specify grants composed of more than one word
 - mysql_user: name=replication password=12345 priv=*.*:"REPLICATION CLIENT" state=present
 
+# Revoke all privileges for user 'bob' and password '12345' 
+- mysql_user: name=bob password=12345 priv=*.*:USAGE state=present
+
 # Example privileges string format
 mydb.*:INSERT,UPDATE/anotherdb.*:SELECT/yetanotherdb.*:ALL