[commands] Update Command documentation (NFC) (#3881)

Add reference to which VendorDep the class is included in.
Add missing OldCommands C++ Documentation (copied from Java).

wpilibNewCommands/src/main/native/include/frc2/command/ConditionalCommand.h

@@ -29,6 +29,8 @@ namespace frc2 {
  * <p>As a rule, CommandGroups require the union of the requirements of their
  * component commands.
  *
+ * This class is provided by the NewCommands VendorDep
+ *
  * @see ScheduleCommand
  */
 class ConditionalCommand