[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/ProxyScheduleCommand.h

@@ -17,6 +17,8 @@ namespace frc2 {
  * all the commands are no longer scheduled.  Useful for forking off from
  * CommandGroups.  If this command is interrupted, it will cancel all of the
  * commands.
+ *
+ * This class is provided by the NewCommands VendorDep
  */
 class ProxyScheduleCommand
     : public CommandHelper<CommandBase, ProxyScheduleCommand> {