[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/SequentialCommandGroup.h

@@ -29,6 +29,8 @@ const size_t invalid_index = std::numeric_limits<size_t>::max();
  *
  * <p>As a rule, CommandGroups require the union of the requirements of their
  * component commands.
+ *
+ * This class is provided by the NewCommands VendorDep
  */
 class SequentialCommandGroup
     : public CommandHelper<CommandGroupBase, SequentialCommandGroup> {