I realize why the original was spelled wrong, volatile is a c++ keyword. Modify to not require learning a misspelling, and note why it can't have that name (#725)

Signed-off-by: Emerson Knapp <eknapp@amazon.com>

rclcpp/include/rclcpp/qos.hpp

@@ -105,8 +105,11 @@ public:
   durability(rmw_qos_durability_policy_t durability);
 
   /// Set the durability setting to volatile.
+  /**
+    * Note that this cannot be named `volatile` because it is a C++ keyword.
+    */
   QoS &
-  volatile();
+  durability_volatile();
 
   /// Set the durability setting to transient local.
   QoS &

rclcpp/src/rclcpp/qos.cpp

@@ -120,7 +120,7 @@ QoS::durability(rmw_qos_durability_policy_t durability)
 }
 
 QoS &
-QoS::volatile()
+QoS::durability_volatile()
 {
   return this->durability(RMW_QOS_POLICY_DURABILITY_VOLATILE);
 }